This study examines the differences which may distinguish systemic lupus erythematosus (SLE) presenting in adult life or childhood. A common database was established, with analysis of clinical, serological and outcome features of a cohort of patients with SLE, with disease diagnosed before the age of 16 (n = 39) or after the age of 16 (n = 165). Disease onset was generally more severe in the childhood-onset patients. Cardiopulmonary disease was more common in the older-onset group, but major haematological manifestations were more frequent in the childhood-onset group. Serologically, anti-DNA, anti-Sm and anti-RNP antibodies and a low C3 were all found more frequently in the younger patients. Twice as many adult-onset cases had died at the time of the last follow-up (10 vs 5%), but this group had been followed for a longer period (average 7.5 yr, S.D. 3.9 for adults vs average 4.8 yr, S.D. 3.2 for children). However, the younger patients were twice as likely (82 vs 40%) to require high-dose prednisone, although the requirement for immunosuppressive agents was similar in the two groups. Clinicians should anticipate that children with SLE have a more severe disease onset than adults in general.