SummaryDifferent types of congenital haemolytic anaemia are reviewed, with special reference to deficiency of glucose-6-phosphate dehydrogenase. Aetiolo-gical, hereditary and clinical aspects are briefly presented. After a short review of the structure and metabolism of the erythrocyte, the pathogenesis of haemolysis is dealt with in some detail : in the presence of certain definite compounds, G-6-PD deficiency leads to a fall in TPNH and reduced glutathion levels. The various theories put forward to explain the mechanisms of haemolysis and the origin of methaemoglobin and Heinz bodies, are discussed.Certain biochemical abnormalities observed in other congenital haemolytic syndromes are also presented.