Methemoglobinemia, a potentially life–threatening condition in which the oxygen–carrying capacity of blood in body tissues is reduced, is a known adverse effect of dapsone. Clinical suspicion of methemoglobinemia can be made when there is cyanosis, saturation gap and chocolate brown color of arterial blood. Here we report a case with classical saturation gap a high index of suspicion for acquired Methemoglobinemia in a patient who is a known case of idiopathic thrombocytic purpura on treatment with Dapsone. Case emphasizes the importance of good history taking and knowledge of drugs likely to cause Methemoglobinemia. Although our patient did not have any signs of peripheral or central cyanosis, Saturation gap on Arterial blood gas analysis suggested the diagnosis of Dapsone induced Methemoglobinemia.