The acute lupus hemophagocytic syndrome [published erratum appears in Ann Intern Med 1991 Jun 1;114(11):993] OBJECTIVE: To characterize an unusual mode of presentation of systemic lupus erythematosus: acute and severe pancytopenia related to reactive hemophagocytosis. DESIGN: Retrospective case series. SETTING: Two general community hospitals in Hong Kong. PATIENTS: Six patients presenting with a reactive hemophagocytic syndrome, identified over a 3.5 year period, diagnosed with systemic lupus erythematosus according to the criteria of the American Rheumatism Association. RESULTS: In addition to severe pancytopenia and marrow hemophagocytosis, other characteristic features were fever, hypocomplementemia, high antinuclear antibody titer, and cutaneous and visceral vasculitis. There was no evidence of an underlying infection. The pancytopenia responded dramatically to treatment with steroids. CONCLUSION: Recognition of the acute lupus hemophagocytic syndrome and distinction from an infection-associated hemophagocytic syndrome is important because it responds well to steroid therapy. The evaluation of patients presenting with a hemophagocytic syndrome should include serologic tests for systemic lupus erythematosus. 