The authors summarize their experience with 174 patients with aplastic anaemia (AA) with particular reference to course, prognostic factors, conversion to other blood disorders, treatment and outcome. Aplastic anaemia was defined as pancytopenia and a hypocellular marrow at some time during the illness. Seven patients terminated with acute leukaemia, 8 developed haematological features of sideroblastic anaemia and 5 showed a red cell membrane defect commonly found in paroxysmal noctural haemoglobinuria. Complete recovery occurred in only 4 patients; 70 others showed a partial remission with some residual haematological abnormality in peripheral blood. Eighteen of these had spontaneous remission. Remission was brief in 27 patients who died; only 32 patients remained in partial remission. Twenty-five per cent. of the patients with AA run an acute course and die within 6 months of the time of diagnosis. The remainder run a subacute or chronic course, punctuated in some cases by a transient remission, but in others by a conversion to other related haematological disorders. An acute course is suggested by a rapid onset of symptoms, a falling neutrophil count, a haemoglobin level less than 5 g/dl and a very low platelet count. A chronic course is likely in those patients who have a slow onset of symptoms, a stable neutrophil count and a Hb level in excess of 5 g/dl. The authors' experience shows that the disease runs either of the 2 courses irrespective of the supportive therapy.