137 patients with severe aplastic anaemia (SAA) were treated in Basel from 1976 to 1986. 34 underwent bone marrow transplantation (BMT) and 103 received antilymphocyte globulin (ALG) therapy. We have analysed the incidence of late haematological complications in both groups of patients. 20 patients treated with ALG developed a late haematological complication. A myelodysplastic syndrome or frank leukaemia occurred in eight and paroxysmal nocturnal haemoglobinuria (PNH) in 13 patients. Nine of the 13 patients with PNH had clinical signs of haemolysis, four only had positive laboratory tests. One patient had PNH and acute leukaemia. The risk of developing a haematological complication increased continuously and reached 57% at 8 years. Neither PNH nor leukaemia occurred in patients treated with BMT. The increased survival rate and the long observation time after ALG therapy have revealed a new perspective of the prognosis of aplastic anaemia. Patients treated with BMT appear to be cured whereas those treated with ALG remain at risk for late complications.