Background & Aims: Histologically assessed hepatocyte ballooning is a key feature discriminating non-alcoholic steatohepatitis (NASH) from steatosis (NAFL). Reliable identification underpins patient inclusion in clinical trials and serves as a key regulatory-approved surrogate endpoint for drug efficacy. High inter/intra-observer variation in ballooning measured using the NASH CRN semi-quantitative score has been reported yet no actionable solutions have been proposed., Methods: A focused evaluation of hepatocyte ballooning recognition was conducted. Digitized slides were evaluated by 9 internationally recognized expert liver pathologists on 2 separate occasions: each pathologist independently marked every ballooned hepatocyte and later provided an overall non-NASH NAFL/NASH assessment. Interobserver variation was assessed and a 'concordance atlas' of ballooned hepatocytes generated to train second harmonic generation/two-photon excitation fluorescence imaging-based artificial intelligence (AI)., Results: The Fleiss kappa statistic for overall interobserver agreement for presence/absence of ballooning was 0.197 (95% CI 0.094-0.300), rising to 0.362 (0.258-0.465) with a ≥5-cell threshold. However, the intraclass correlation coefficient for consistency was higher (0.718 [0.511-0.900]), indicating 'moderate' agreement on ballooning burden. 133 ballooned cells were identified using a ≥5/9 majority to train AI ballooning detection (AI-pathologist pairwise concordance 19-42%, comparable to inter-pathologist pairwise concordance of between 8-75%). AI quantified change in ballooned cell burden in response to therapy in a separate slide set., Conclusions: The substantial divergence in hepatocyte ballooning identified amongst expert hepatopathologists suggests that ballooning is a spectrum, too subjective for its presence or complete absence to be unequivocally determined as a trial endpoint. A concordance atlas may be used to train AI assistive technologies to reproducibly quantify ballooned hepatocytes that standardize assessment of therapeutic efficacy. This atlas serves as a reference standard for ongoing work to refine how ballooning is classified by both pathologists and AI., Lay Summary: For the first time, we show that, even amongst expert hepatopathologists, there is poor agreement regarding the number of ballooned hepatocytes seen on the same digitized histology images. This has important implications as the presence of ballooning is needed to establish the diagnosis of non-alcoholic steatohepatitis (NASH), and its unequivocal absence is one of the key requirements to show 'NASH resolution' to support drug efficacy in clinical trials. Artificial intelligence-based approaches may provide a more reliable way to assess the range of injury recorded as "hepatocyte ballooning"., Competing Interests: Conflicts of interest AJS is president of Sanyal Bio. He reports consultancy fees from Merck, Regeneron, Alnylam, Genentech, Amgen, Pfizer, Novo Nordisk, Eli Lilly, Boehringer Ingelhiem, Inventiva, Madrigal, Malinckrodt, Salix, Genfit, Hemoshear, Histoindex, Siemens, Gilead, NGM, Terns, Rivus, Endiva, 89Bio, Akero, Blade, Novartis, Axcella, Intercept; grant funding from Intercept, Gilead, Bristol Myers Squibb, Merck, Pfizer, Novo Nordisk, Astra Zeneca, Boehringer Ingelhiem, Eli Lilly, Viking, Madrigal, Akero, Hanmi; stock/stock options from Genfit, Tiziana, Indalo, Exhalenz, Hemoshear; royalties from Elsevier. AW reports fees from Clinnovate. BNT reports consultancy fees from Histoindex. CDG reports consultancy fees from HistoIndex, NGMBio, CymaBay and Madrigal. CDG has or has had consulting agreements with the following companies: HistoIndex, NGM, CymaBay, Madrigal and 89Bio. DGT reports consultancy for Intercept Pharmaceuticals Inc, Allergan plc, Verily Life Sciences LLC, Cirius Therapeutics Inc, Alimentiv Inc, Clinnovate Health UK Ltd, ICON Clinical Research Ltd and grants from Histoindex Pte Ltd. DT is an employee of, and holds stock in, Histoindex. EC and RY are employees of Histoindex. EMB has been on Advisory Board for Pfizer Ltd. And served as a consultant for Arrowhead, CymaBay, Intercept Pharmaceuticals, Medpace, Perspectum Diagnostics and Histoindex. Paid slide evaluation for CymaBay, Medpace and Perspectum Diagnostics. MR reports consultancy fees from Alnylam, Amgen, AMRA, BMS, Boehringer Ingelheim, Centara, Coherus, Enanta, Galecto, Intercept Pharmaceuticals, Madrigal, NGM, Biopharmaceuticals, Novo Nordisk, Pfizer, Fractyl, Gelesis, Siemens, Thetis, Terns, Rivus, 3vBio (Sagimet), 89Bio and Novartis, Immuron, Merck, Taiwan J. QMA is coordinator of the IMI2 LITMUS consortium, which is funded by the EU Horizon 2020 programme and EFPIA. This multi-stakeholder consortium includes industry partners. He reports research grant funding from Allergan/Tobira, AstraZeneca, GlaxoSmithKline, Glympse Bio, Novartis Pharma AG, Pfizer Ltd., Vertex; consultancy on behalf of Newcastle University for 89Bio, Allergan/Tobira, Altimmune, AstraZeneca, Axcella, Blade, BMS, BNN Cardio, Cirius, CymaBay, EcoR1, E3Bio, Eli Lilly & Company, Galmed, Genentech, Genfit, Gilead, Grunthal, HistoIndex, Indalo, Intercept, Inventiva, IQVIA, Janssen, Madrigal, MedImmune, Medpace, Metacrine, NGMBio, North Sea Therapeutics, Novartis, Novo Nordisk A/S, PathAI, Pfizer Ltd., Poxel, ProSciento, Raptor Pharma, Roche, Servier, Terns, The Medicines Company, Viking Therapeutics; and speaker fees from Abbott Laboratories, Allergan/Tobira, BMS, Clinical Care Options, Falk, Fishawack, Genfit SA, Gilead, Integritas Communications, Kenes, MedScape. SAH reports grants from Akero, Axcella, Cirius, CiVi, CymaBay, Enyo, Galectin, Galmed, Genfit, Gilead Sciences, Hepion, Hightide, Intercept, Madrigal, Metacrine, NGM, Northsea, Novartis, Novo Nordisk, Poxel, Sagimet, Viking; consultancy fees from AgomAB, Akero, Alentis, Alimentiv, Altimmune, Axcella, Boston Pharma, B Riley, BVF Partners, Canfite, CiVi, Corcept, CymaBay, Echosens, Enyo, Fibronostics, Foresite, Fortress, Galectin, Genfit, Gilead, GNS, Hepion, Hightide, HistoIndex, Inipharm, Intercept, Ionis, Kowa, Madrigal, Metacrine, Microba, NGM, NorthSea, Novartis, Novo Nordisk, Nutrasource, Piper Sandler, Poxel, Prometic Pharma, Sagimet, Sonic Incytes, Terns, Viking; and stock holdings or stock options in Akero, Chronwell, Cirius, Galectin, Genfit, Hepion, HistoIndex, Metacrine, NGM, NorthSea, PathAI, Sonic Incytes. VR reports consultancy fees from Novo-Nordisk, Intercept, NGM, Theratechnologies, Galmed, Madrigal, Hanmi, Astra-Zeneca. VT reports research grants from Gilead sciences and consulting fees from Novo-Nordisk, Intercept, NGM, Theratechnologies, Galmed, Madrigal, Hanmi, Astra-Zeneca. The other authors report no relevant conflicts of interest. Please refer to the accompanying ICMJE disclosure forms for further details., (Copyright © 2022. Published by Elsevier B.V.)