A randomized, prospective, open-label, treatment versus no treatment community-based clinical trial was conducted to evaluate the safety and efficacy of clofazimine as prophylaxis for disseminated Mycobacterium avium complex (MAC) infection in patients with human immunodeficiency virus (HIV) disease. Subjects were 110 patients with a first episode of Pneumocystis carinii pneumonia 2-4 months before enrollment or CD4 lymphocyte counts < or = 100/mm3; they were randomized to receive 50 mg of clofazimine daily or no treatment. Seven patients randomized to clofazimine developed disseminated MAC infection, compared with 6 patients receiving no treatment. Seventeen patients died: 9 in the treatment group and 8 receiving no treatment. Clofazimine at a dose of 50 mg/day is well tolerated by patients with HIV disease. Reduction in CD4 lymphocyte count to < 50/mm3 is a significant predictor of the development of disseminated MAC infection.