Objective. To examine the evolution of psychosocial aspects of health-related quality of life in rheumatoid arthritis (RA) patients, and to identify their predictors. Methods. All patients within a Swiss RA cohort and a US RA cohort who completed a Short Form 36 (SF-36) scale at least twice within a 4-year period were included. The primary outcome was psychosocial health as measured by the mental component summary (MCS) score of the SF-36. The evolution of this outcome over time was analyzed using structural equation models, which distinguish between the stable, the variable, and the measurement error components of the outcome's variance. Results. A total of 15,282 patients (48,323 observations) were included. MCS scores were mostly stable over time (between 69% and 75% of the variance was not due to measurement error). The variable component of the SF-36 was mostly due to fluctuations at the moment of measurement and not to a global time trend of psychosocial health. Pain was the most important predictor of both the stable and variable components of psychosocial health, explaining 44% of the observed psychosocial health variance. Conclusion. This large cohort study demonstrates that pain is the most important predictor of a patient's psychosocial health in RA patients. This suggests that physicians should place greater emphasis on pain management.