In a prospective, noncomparative study, seven patients with mild Pneumocystis carinii pneumonia, characterized by room air arterial PO2 greater than 60 mm Hg at the time of presentation, were treated with dapsone alone at a dose of 200 mg daily. Two of the seven patients required mechanical ventilation for respiratory failure on day 5 of dapsone therapy; both died. Four patients experienced major side effects during dapsone therapy. None of the seven patients successfully completed a full course of therapy with dapsone. We conclude that high-dose, single-agent dapsone is not suitable for further study as therapy for Pneumocystis carinii pneumonia.