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Objective: To estimate the rate of change in functional limitations for patients with rheumatoid arthritis (RA) as a function of age, duration of illness, and sex., Methods: Patients with RA (n = 700) aged 21-65 years in 1988 were interviewed yearly for 6 years in The National Rheumatoid Arthritis Study. Functional limitations scores based on a Rasch measurement model of 20 Health Assessment Questionnaire items were analyzed in mixed-effects models to estimate the rate of change in functional ability as a function of age, duration of illness, sex, and interactions., Results: Models for both patient age and duration of illness significantly predicted limitations in functional ability for men and women. The model for age included a significant cubic effect; the model for duration of illness included a significant linear effect only. Sex was significant in both models and no interactions were significant in either model. The AIC index of fit, an indicator of the information value of the model, favored the model for duration of illness over the model for age. While both models showed higher levels of functional limitations in women than men, the rate of change for women was similar to men., Conclusion: Limitation in functional ability in RA progressed in a linear manner with duration of illness and progressed at the same rate for both men and women, but functional limitations were greater for women.