The efficacy, toxicity and cost of orally administered dapsone (50-100 mg/day) for prophylaxis of Pneumocystis carinii pneumonia (PCP) were evaluated in 30 patients with AIDS or AIDS-related complex (ARC). Six patients received primary and 24 secondary prophylaxis. Ten patients received a maximum dose of 100 mg/day and 20 a maximum of 50 mg/day for a median duration of 19 weeks; 22 of the 30 patients continue to receive prophylaxis as of May 1989. Four patients have died, none of pneumocystis infection. One patient with AIDS suffered a mild relapse while receiving 50 mg/day. Hematologic toxicity was mild and could not be definitively attributed to dapsone therapy; rash due to dapsone was documented in two patients. A review of 33 patients at our institution with a history of PCP who received no prophylaxis demonstrated seven relapses, three of which were fatal. Cost analysis revealed a significant advantage for oral dapsone over aerosolized pentamidine.