124 adult patients with acquired aplastic anaemia (A.A.) were studied. 54 presented as severe A.A. (mean survival of the whole group, 2.7 months and in those who died, 2.5 months; mortality 88.8%) while 70 were mild (mean survival of the whole group, 27.2 months and in those who died, 22.7 months; mortality, 28.5%). Sixty-four did not live long enough to have adequate therapeutic trials or developed severe hepatic failure which made it impossible to continue treatment. The effectiveness of 78 therapeutic periods of treatment in the 60 evaluable cases was analyzed as to aetiology and severity. A response was obtained in 31 (40%), of which 24 were with oxymethalone; 17 showed improvement after 6 months of treatment. No conclusions could be drawn as to the effectiveness of methalone, cyclophosphamide and prednisone because of the limited number of treatments. In severe A.A. only one of 13 treatments was effective, whereas 30 of the 65 used in mild A.A. gave a response. Oxymethalone was used in 23 out of 41 (56%). The aetiology, sex and age appeared to have no influence on the response of the 60 patients analyzed. The only factor which appears useful for prognosis as to the effectiveness of treatment is the severity of the case.