Ankylosing spondylitis (AS) with juvenile onset (under 17 years of age) is not infrequent. Thirty-six cases were studied, amounting to 18% of patients hospitalized between 1977 and 1981. The following criteria were used for diagnosis: radiologic sacroiliitis (typical AS), presence of HLA B27 and/or pelvic or vertebral clinical manifestations (possible AS). 31 patients (85%) were boys. Mean age at onset was 12.3 +/- 2.8 years. In three cases, AS was found in a member of the family of the propositus and in one case there was cutaneous psoriasis. Usually (29 cases) onset was in the lower limbs: arthritis of the knee (14 cases), hip (9 cases), ankle (7 cases) or painful heel (4 cases). During the course (with a mean follow-up of 11.2 +/- 7 years), 35 patients exhibited peripheral joint diseases and 25 had axial involvement. Ocular involvement was present in 5 cases. 10 patients had a modification of respiratory function. Radiologic sacroiliitis was found in 31 patients but with a delay of 5.3 +/- 2.6 years. Vertebral radiologic lesions were only seen in 11 patients. Radiologic hip involvement was frequent (20 cases) with complete destruction in 6 patients. Erosion and ossification of the calcaneum were observed in 15 cases. The ESR was above 20 mm/first hour in 26 cases (72%). 81% of these patients were HLA B27 positive. Functional prognosis was good: 16 patients (51.6%) led an almost normal life, 6 were bedridden (Steinbrocker's grade IV), 3 had severe impairment (grade III) and 6 had slight impairment (grade II).(ABSTRACT TRUNCATED AT 250 WORDS)