Methemoglobinemia results from the oxidation of the ferrous iron in hemoglobin to the ferric iron state. Methemoglobin is incapable of carrying O2, and high levels may impact on O2 delivery to the tissues. Methemoglobinemia may result from congenital deficiencies of enzymes that normally convert methemoglobin to hemoglobin, alterations in the hemoglobin molecule itself or, most commonly, from the ingestion of medications or toxins that oxidize the ferrous iron of hemoglobin. Several issues must be considered when anesthetizing patients with methemoglobinemia, including the potential for decreased O2 delivery, which may be exacerbated by intraoperative blood loss and anemia, interference with normal intraoperative monitoring devices, and the potential for medications to cause or exacerbate methemoglobinemia. We describe a patient with acquired methemoglobinemia from dapsone therapy who required anesthetic care for shoulder arthroscopy. The patient's drug-induced methemoglobinemia was diagnosed intraoperatively during previous anesthesia on the basis of discrepancy between the O2 saturation noted by pulse oximetry and that obtained from arterial blood gas analysis. Anesthetic care for patients with methemoglobinemia is discussed and a review of methemoglobinemia presented.