Seventy-four hospitalized patients with ankylosing spondylitis (HOSP AS) were compared clinically with 27 AS patients found in an epidemiological study (EPI AS). The distribution of radiographic changes, initial symptoms and age at onset of AS, frequency of HLA-B27 and acute anterior uveitis were similar in the 2 groups. The EPI AS patients had less restricted spinal and chest mobility than the hospitalized patients. Restriction of mobility thus appears to be an important factor for referral of patients with AS to hospital. A higher frequency of employment among EPI AS patients was found.