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Objective: Precision medicine in rheumatoid arthritis (RA) requires a good understanding of treatment outcomes and often collaborative efforts that call for data harmonisation. We aimed to describe how harmonisation across study cohorts can be achieved and investigate how the observed proportions reaching remission vary across remission criteria, study types, disease-modifying antirheumatic drugs (DMARDs) and countries, and how they relate to other treatment outcomes., Methods: We used data from eight existing large-scale, clinical RA registers and a pragmatic trial from Sweden, Denmark and Norway. In these, we defined three types of treatment cohorts; methotrexate monotherapy (as first DMARD), tumour necrosis factor inhibitors (TNFi) (as first biological DMARD) and rituximab. We developed a harmonised study protocol defining time points during 36 months of follow-up, collected clinical visit data on treatment response, retention, persistence and six alternative definitions of remission, and investigated how these outcomes differed within and between cohorts, by treatment., Results: Cohort sizes ranged from ~50 to 22 000 patients with RA. The proportions reaching each outcome varied across outcome metric, but with small to modest variations within and between cohorts, countries and treatment. Retention and persistence rates were high (>50% at 1 year), yet <33% of patients starting methotrexate or TNFi, and only 10% starting rituximab, remained on drug without other DMARDs added and achieved American Congress of Rheumatology/European Alliance of Associations for Rheumatology or Simplified Disease Activity Index remission at 1 year., Conclusion: Harmonisation of data from different RA data sources can be achieved without compromising internal validity or generalisability. The low proportions reaching remission, point to an unmet need for treatment optimisation in RA., Competing Interests: Competing interests: SS is a part-time employee at deCODE genetics. E-MH has received institutional grants or contracts from Novo Nordic Foundation, Danish Rheumatism Association, Danish Regions Medicine Grants, Rocke, Novartis or Abbvie, payment or honoraria for lecture, presentations, speakers bureaus, manuscript writing or educational events from Sobi, support from attending meetings and/or travel from Pfizer, Sobi and Abbvie. participated on data safety monitoring board or advisory board for Novartis, Abbvie and Sanofi and is principal investigator in trials by SynACT Pharma, site principal investigator in trials by AbbVie, Novartis, Novo and Sanofi. HBH has received honoraria for lectures from AbbVie, Novartis, Lilly and UCB, and participated on advisory board for AbbVie, UCB and Novarties. Karolinska Institutet has entered into agreements with the following companies, with JA as PI: Abbvie, BMS, Eli Lilly, Galapagos, Janssen, Pfizer, Roche, Samsung Bioepis and Sanofi. BG has received research grant from Pfizer, AbbVie, BMS and Sandoz. The remaining authors have nothing to declare., (© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)