Objective. To compare those who were finally included in a large well-defined sample of individuals with rheumatoid arthritis (RA) at target for a physical activity (PA) trial with those who were not. Methods. In total, 3,152 individuals answered questionnaires on sociodemographic, disease-related, and psychosocial factors and PA levels. The differences between individuals making it to the baseline assessments and those who did not were analyzed in 3 steps. Results. In a first step, 1,932 individuals were eligible for the trial if they were interested in articipating, not physically active enough, and fluent in Swedish and if they were not participating in any other study. The participants were mainly younger women, had higher education and income, were more likely to live with children, and had better support for exercise and had higher outcome expectations of PA than the 1,208 ineligible individuals. In a second step, the 286 individuals accepting participation had higher income and education, more support for exercise, less fear-avoidance beliefs, and higher outcome expectations of PA than the 1,646 individuals declining participation. In a third step, the 244 individuals assessed at baseline reported less fatigue than the 42 withdrawing before assessment. Conclusion. To our knowledge, this is the first study describing the entire selection procedure, from a target sample for a PA trial to the sample assessed at baseline, in individuals with RA. Factors other than those related to the disease seemed to mainly determine participation and largely resembled determinants in the general population. Sociodemographic and psychosocial factors should be recognized as important for PA in people with RA.