Sex differences in the prevalence of autoimmune diseases such as rheumatoid arthritis (RA) are well described, but the literature is not as clear about sex differences in RA disease course and prognosis. A recent study from a very large cross-sectional international cohort demonstrated slightly worse levels of disease activity and function in female patients with RA, compared with men. These findings are discussed in the context of our evolving knowledge of sex differences in the expression of this prototypic autoimmune disease, both in terms of the actual disease activity level, the effects that the disease has on physical function, and our ability accurately to measure these aspects.