The incidence, prevalence, and outcome of systemic lupus erythematosus (SLE) were examined in a large, well-defined population for which extensive records of complete medical care are available. The prevalence was 1:1,969; in women aged 15 to 64 years, approximately 1:700; in black women 15 to 64, 1:245. Mortality was low: only five deaths among 70 patients in whom SLE was freshly diagnosed occurred during an eightyear period. The projected ten-year survival rate exceeds 90%. The prevalence of clinically important renal disease was only 11%. Previous estimates of the prevalence of renal disease in SLE have been based on skewed series. Several clinical patterns of SLE were discerned: least common was the violent, rapidly fatal one; most patients experienced a benign, chronic course or intermittently stuttering illness; some had only one or a few brief episodes of the disease.