26 patients, 13 male and 13 female, with paroxysmal nocturnal haemoglobinuria (PNH) are described. The diagnosis was based on the finding of a positive Ham’s test. PNH developed in 4 patients with aplastic anaemia, and 3 patients with established PNH developed marrow hypoplasia during the course of the disease. In 2 cases autoimmune haemolysis was also present; this association has not been described previously. The majority of patients presented with anaemia and dark urine, or with evidence of thrombosis. A high index of suspicion was needed to avoid missing the diagnosis. Haemolytic crises were usually precipitated by infection, and renal failure requiring dialysis sometimes resulted; a positive direct antiglobulin test was often found at times of increased haemolysis. Thromboses were the most frequent complication, and when intra-abdominal vessels were affected, pain was particularly troublesome. The disease had a widely variable course; 4 patients made a complete recovery and 10 died, 8 from thrombotic complications and 2 from infections associated with marrow hypoplasia. Survival ranged from 1 year to 30 years and the median survival in those who died was 3.5 years.