This report describes a case of methemoglobinemia in association with dapsone therapy. The patient, an immunocompromised child with chronic immune thrombocytopenic purpura, presented with fever, cough, perioral cyanosis, bilateral lower lobe rales, and low O2 saturation by pulse oximetry (89%). His medications included prednisone and rituximab for chronic immune thrombocytopenic purpura, and dapsone for Pneumocystis carinii pneumonia prophylaxis. Because of his lack of dyspnea and tachypnea, and the temporal association of his perioral cyanosis with the initiation of dapsone therapy, a methemoglobin (MetHb) level was obtained and found to be elevated at 9.6%. The authors discuss the mechanism and treatment of methemoglobinemia secondary to dapsone. They also stress the importance of monitoring for signs and symptoms of methemoglobinemia in immunocompromised patients on dapsone therapy for P. carinii pneumonia prophylaxis.