Methaemoglobin is haemoglobin in which the sixth coordination position of heme iron is in the ferric state, having lost an unpaired electron. In normal human blood the level of methaemoglobin is 1% or less. This stable level results from an equilibrium between the rate at which ferric heme is formed and the rate at which it is reduced back to the ferrous state. Congenital methaemoglobinaemia caused by a deficiency or absence of methaemoglobin reductase represents a disturbance of this equilibrium wherein the methaemoglobin formed at physiological rate is not efficiently reduced. A chronically elevated level of oxidized haemoglobin results. Methaemoglobin is incapable of binding oxygen. Instead of the bright red colour imparted by oxyhaemoglobin, blood containing more than 10% methaemoglobin is dark brown. A concentration exceeding 1.5 g/d1 gives rise to cyanosis. The majority of patients with congenital enzymopenic methaemoglobinaemia are asymptomatic despite the intense cyanosis. This is a very important differentiating point from the cyanosis of cardiopulmonary causes. Methaemoglobin reductase deficiency is uncommon. Most proven cases had been in persons of European descent with sporadic reports of non-Caucasoid patients. We report here a Chinese patient whom we believe is the first in South East Asia to have methaemoglobin reductase deficiency unequivocally documented.