BACKGROUND: To assess the risks of malignancies, infections and autoimmune diseases in patients with rheumatoid arthritis (RA) treated with abatacept compared with other biologic (b) disease-modifying antirheumatic drugs (DMARDs) or conventional synthetic (cs)DMARDs, in a US-wide observational RA cohort METHODS: Data were reviewed from patients (≥ 18 years) with RA who were registered with FORWARD, the National Databank for Rheumatic Diseases, and who initiated abatacept, other bDMARDs or csDMARDs between 2005 and 2015. Patients who switched treatment during the study could be allocated to more than one group. The incidence rates (IRs) by treatment were calculated for malignancies, hospitalized infections and autoimmune diseases identified by six monthly questionnaires and medical records. The hazard ratios (HRs) (95% confidence intervals [CIs]) for all outcomes with abatacept compared with other bDMARDs or csDMARDs were determined using marginal structural models adjusted for clinical confounders. RESULTS: In the study sample, 1496 initiated abatacept, 3490 initiated another bDMARD and 1520 initiated a csDMARD. The risk of malignancies with abatacept was not statistically significant versus other bDMARDs (HR [95% CI)] 1.89 [0.93, 3.84]) or versus csDMARDs (HR [95% CI] 0.93 [0.20, 4.27]). Patients receiving abatacept versus other bDMARDs were at a lower risk of hospitalized infections (HR [95% CI] 0.37 [0.18, 0.75]); the risk versus csDMARDs was lower with wide CIs (HR [95% CI] 0.31 [0.09, 1.05]). The relative risks for psoriasis were similar between treatment groups (HR [95% CI] 1.46 [0.76, 2.81] and HR [95% CI] 2.05 [0.59, 7.16] for abatacept versus other bDMARDs and versus csDMARDS, respectively). The IR (95% CI) of severe infusion/injection reactions was lower with abatacept compared with other bDMARDs (1.57 [1.11, 2.17] vs 2.31 [1.87, 2.82] per 100 patient-years, respectively). CONCLUSIONS: In this analysis, abatacept was well tolerated and did not result in an overall increased risk of malignancies, infections or autoimmune diseases compared with other bDMARDs or csDMARDs.