Dapsone, a sulfone antibiotic, has been increasingly used in solid-organ transplant recipients for the primary prevention of Pneumocystis carinii pneumonia, especially in patients with documented sulfa allergy. A known side effect of dapsone therapy, however, is methemoglobinemia, a condition leading to impaired tissue oxygen delivery. This report documents two cases of dapsone-induced methemoglobinemia in patients after solid organ transplantation with emphasis on the importance of clinical recognition and benefits of treatment. Further, the pathophysiology and causes of this condition are extensively reviewed.