The haematological data in a 9-year-old Dutch child suffering from an unusual new variant of glucose-6-phosphate isomerase deficiency (GPI Utrecht) are discussed. Symptoms and signs differ in many respects from those generally observed in GPI deficiency, as mental retardation, drug sensitivity and increased susceptibility to infections were present. Extensive studies on mechanisms involved in drug-sensitive haemolytic anaemia did not reveal its causes. Though the defect was generalized, no disturbance in granulocyte and thrombocyte functions were detected.