Whether physical functioning in patients with rheumatoid arthritis (RA) differs from that in patients with ankylosing spondylitis (AS) is presently uncertain. Such a comparison poses challenges, not only because the two diseases differ in the domains of functioning affected, but also because of the different instruments used to measure functional limitations. Limiting our analysis to studies using similar self-report questionnaires, we examined published observational studies of unselected cohorts of patients with RA and patients with AS to compare and contrast the severity of functional limitations. Available studies from a few direct comparisons, and mostly indirect comparisons, suggested that patients with RA are generally more severely limited in physical functioning throughout the disease course than patients with AS. Since most studies did not adjust adequately for potentially important confounders, such as age, gender, comorbidity, and disease duration, reported differences in functional disability between patients with RA and patients with AS must be interpreted cautiously.