Aplastic anemia is a disease characterrized by anemia, neutropenia and thrombocytopenia with a pathological basis of bone marrow disfunction. Despite advances in medical knowledge, it has remained a disease with a high fatality rate and a disease which possesses many problems in management. The purpose of this paper is to describe and discuss the factors regulating the prognosis and treatment on a group of 181 patients with primary and secondary aplastic anemia seen during 21 year period in Hiraki Medical Clinic since 1951. The statistical analysis on age, sex, hematological data and outcome was performed with a computer (NEAC-2200-500). The patients consisted of 163 cases of primary and 18 cases of secondary aplastic anemia. The patients with pancytopenia following treatment with cytotoxic drugs or radiation therapy were excluded. Concerning, sex, there were 104 males and 73 females, with ages ranging from 5-76 years at their onset. The overall survival rate was 40.6 % ; 35.1 % in males and 54.1 % in females. As to the age distribution, one found two major and minor peaks at ages of 10 and 60, respectively. The advanced age group had higher mortality rate and shorter survival period than in younger age group. The survival period of the patients with fatal outcome was 34.7 months in average from onset to death and that of the alive patients was 134.2 months in average. As to the relationship between the survival period and outcome, the patients with longer survival period showed lower mortality rate. Life span of the 50 % of patients with aplastic anemia was found to be within 4.25 years. And, among dead cases, approximately 42 % of the patients survived for less than one year. To estimate the efficacy of the treatment, the medication schedules were classified into five groups: "C.S. + ACTH + A.S." group who were treated with corticosteroid, adrenocorticotropin and anabolic steroid; "C.S. + ACTH" group; "C.S." group; "ACTH" group and the 5th group treated with only blood transfusion and vitamins. No significant difference could be obtained among these subdivided groups, except for a tendency that hematological findings were improved gradually in the "C.S. + ACTH" group. In conclusion, age at the onset of the disease and the degree of abnormality in hematological data at the first examinations might be most important factors regulating the prognosis. Since the mortality rate is high especially in the patients within one year from the onset the carefully planned treatments and prevention of complications, such as hemorrhage and infection, with minimized blood transfusion are necessary during this period in order to obtain more favorable prognosis.