To gain a better understanding of systemic lupus erythematosus (SLE) in Puerto Ricans we studied the clinical and serologic manifestations in a cohort of 134 patients. The female to male ratio was 18:1. Mean age at diagnosis was 32 +/- 12 y. The mean duration of disease and follow-up were 7.4 +/- 6.0 and 5.8 +/- 6.0 years respectively. Mortality was 3%. Photosensitivity (76.9%) and malar rash (71.9%) were the most common clinical manifestations. Arthritis was observed in 67.5% of patients. Anemia was seen in 67.2% of patients, but only 12.7% had autoimmune hemolytic anemia. Leukopenia (41.8%) and lymphopenia (64.9%) were also common. Serositis was observed in only 28%. Severe kidney damage such as nephrotic syndrome (14.2%) or renal failure (4%) was infrequent. Cardiovascular (12.7%) and neurologic (9.0%) manifestations were also uncommon. Antinuclear antibodies (ANA) were detected in 93.3%, anti-dsDNA antibodies in 54.5%, anti-Ro antibodies in 30.1% and anti-La antibodies in 14.2%. Low C3 and low C4 were observed in 38.3% and 35.7% respectively. This study suggests that Puerto Ricans with SLE present a mild form of disease predominantly manifested by cutaneous, musculoskeletal and hematologic involvement, but low prevalence of major organ damage and low mortality.