We studied prospectively the clinical and radiographic features of sacroiliac and spinal involvement in 20 patients with seronegative enthesopathy and arthropathy. This group was compared with 25 patients with a polyarticular onset form of juvenile rheumatoid arthritis (JRA) and 28 with definite ankylosing spondylitis (AS) of juvenile onset. A significant increasing proportion of patients with seronegative enthesopathy and arthropathy developed back complaints and radiographic sacroiliitis fulfilling the diagnostic criteria for AS from the 3rd-5th year of disease (47.1-75.0%) and thereafter (92.3%). Back complaints were rarely seen in JRA and, furthermore, sacroiliitis of the AS type nerve occurring in this group. There were no significant differences between the group with seronegative enthesopathy and arthropathy and juvenile AS, either at onset or through the years. Clinical and radiographic assessment of axial involvement in children at risk should include a careful analysis of symptoms, periodical measurements of the spinal flexion and, starting from the 3rd year, radiographs of the pelvis.