HLA antigens were determined in 38 patients with juvenile chronic polyarthritis (JCP) and in 1,000 normal controls. The incidence of the antigen B27 in JCP (55.3 per cent) was higher than in the controls (6.7 per cent). Patients with JCP could be further subdivided into four groups: (1) JCP evolving to ankylosing spondylitis (AS) (N = 3); (2) JCP with sacroiliitis (SI) (N = 18); (3) JCP without SI (N = 10); and (4) juvenile rheumatoid arthritis (JRA) characterized by positive seroloty (N = 7). Groups 1 and 2 had a high incidence of B27 (20/21), as contrasted to groups 3 and 4 (1/17). The sex distribution in groups 1 and 2 was similar to that found in AS in adults, whereas in groups 3 and 4 it was similar to that found in rheumatoid arthritis in adults. It is concluded that if B27 positives develop JCP, they also have a high risk of developing SI and acute uveitis. The authors propose using the term Still's disease for the overall group of children presenting with JCP before the age of 16 years. Subsequent follow-up permits classification into juvenile AS, JCP with SI, JCP without SI, or as JRA.