Introduction: Immune effector cell-associated neurotoxicity syndrome (ICANS) is a common side-effect of chimeric antigen receptor T-cell (CAR-T) therapy, with symptoms ranging from mild to occasionally life-threatening. The neurological, cognitive, psychiatric and psychosocial sequelae of ICANS are diverse and not well defined, posing a challenge for diagnosis and management. The recovery trajectory of the syndrome is uncertain. Patients are rarely examined in this population pretherapy, adding a layer of complexity to specifying symptoms pertinent solely to CAR-T treatment. We present a protocol of a prospective longitudinal research study of adult patients in a single Australian haematology service undergoing CAR-T therapy. The study will describe neurocognitive features specific to ICANS, characterise the underlying syndrome, capture recovery, identify predictors of differential postinfusion outcomes and determine a set of cognitive instruments necessary to monitor patients acutely., Methods and Analysis: This is a prospective longitudinal study that comprises neuropsychological and neurological examinations occurring prior to CAR-T, during the acute post-treatment period, 28 days, 6 months and 12 months post infusion. Data will be sourced from objective psychometric measures, clinical examinations, self-report questionnaires of psychopathology and accounts of subjective cognitive complaint., Ethics and Dissemination: This study aims to guide diagnosis, management and monitoring of neurocognitive features of CAR-T cell therapy. Results of this study will be disseminated through publication in peer-reviewed journals and presentations at scientific conferences. All procedures involving human subjects/patients were approved by the Peter MacCallum Cancer Centre Human Research Ethics Committee (21/145)., Competing Interests: MAA reports honoraria from AstraZeneca, Janssen, Abbvie, Beigene, Takeda, CSL, Novartis, Kite, Gilead and Roche. Employee of the Walter and Eliza Hall institute which receives Milestone payments in relation to venetoclax. CS reports no disclosures. SvdL reports honoraria from Kite. IR served on scientific advisory boards, received conference travel support and/or speaker honoraria from Roche, Novartis, Merck and Biogen. IR is supported by MS Australia and the Trish Multiple Sclerosis Research Foundation. SR reports no disclosures. FD reports no disclosures. SL reports honoraria from Otsuka and Lundbeck. She has received research support from the National Health and Medical Research Council and Royal Melbourne Hospital. MDowling reports honoraria and conference support from Kite and Gilead and honoraria from Novartis. MDowling also receives royalties from Abbvie in relation to venetoclax via the Walter and Eliza Hall institute. MDickinson reports advisory boards, research funding from Novartis, Kite, BMS and Gilead. TK served on scientific advisory boards for MS International Federation and World Health Organisation, BMS, Roche, Janssen, Sanofi Genzyme, Novartis, Merck and Biogen, steering committee for Brain Atrophy Initiative by Sanofi Genzyme, received conference travel support and/or speaker honoraria from WebMD Global, Eisai, Novartis, Biogen, Roche, Sanofi-Genzyme, Teva, BioCSL and Merck and received research or educational event support from Biogen, Novartis, Genzyme, Roche, Celgene and Merck. SJH reports the following disclosures: AbbVie: consultancy, advisory board; Amgen: consultancy, honoraria, advisory board, research funding; Celgene: consultancy, honoraria, advisory board, research funding; CSL Bering: honoraria; GSK: consultancy, research funding, advisory board; Janssen Cilag: consultancy, honoraria, advisory board, research funding; Novartis: consultancy, honoraria, advisory board, research funding; Roche/Genetec: consultancy, honoraria, advisory board; Takeda: consultancy, honoraria, advisory board; Haemalogix: scientific advisory board, research funding; Sanofi: consultancy/advisory role; Terumo: consultancy/advisory role/expert testimony. CM has received conference travel support from Merck, Novartis and Biogen. He has received research support from the National Health and Medical Research Council, Multiple Sclerosis Australia, The University of Melbourne, The Royal Melbourne Hospital Neuroscience Foundation and Dementia Australia., (Copyright © Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)