We studied 87 Mexican mestizo patients (82 men and 5 women) with definite ankylosing spondylitis (AS) with particular reference to juvenile and adult onset types. HLA-B27 was present in 32 of 38. Forty-seven patients (54.0%) had onset before the age of 16 years and 40 (46.0%) thereafter. By the end of the 1st year of disease, main features included spinal involvement in 44 (50.6%), peripheral arthropathy in 57 (65.5%) and enthesopathy in 41 (47.1%). Frequency of these increased up to 100.0, 79.3 and 64.4%, respectively, through the course of the disease. Peripheral arthritis and/or enthesopathy occurred in 89.4 and 63.1% of juveniles and 37.5 and 27.5% of adults, respectively, while lumbar pain and/or stiffness occurred in 23.4% of the former and 82.5% of the latter during the first year of disease. Additional findings were high erythrocyte sedimentation rate, anemia and hypergammaglobulinemia. Uveitis was the commonest extraarticular manifestation occurring in 20.6%. Our data suggest that the clinical pattern of AS in our patients was influenced by both age at onset and sex distribution of the disease.