Although less frequently, ankylosing spondylitis (AS) may also occur during childhood. The present report describes the clinical features in 24 patients with AS of juvenile onset and its comparison with those found in 71 patients with adult onset AS. A slightly increased prevalence of female patients was found in the juvenile onset group. Peripheral joint involvement as a mono- or oligoarticular arthritis was significantly more frequent in juvenile onset patients, not only as a mode of onset but also during the course of the disease. 5.4 years after onset all these patients had developed axial involvement. Extra-articular manifestations occurred with similar frequency in juvenile and adult onset patients. One juvenile patient had associated multiple sclerosis; one juvenile and one adult onset patient developed amyloidosis. Impairment of functional capacity was more severe in the juvenile onset group; hip joint involvement was closely related to a poorer prognosis.