Mental data from 80 A-bomb survivors were available during a 20-year period 1973-1992. Types of A-bomb survivors were classified into (1) directly exposed A-bomb survivors, (2) A-bomb survivors living in the United States, (3) those living in prefectures other than Nagasaki, (4) ex-soldiers, (5) A-bomb survivors having family problems and others, (6) the demented elderly, (7) the alcoholic, and (8) others. Mental problems were judged as psychogenic, endogenous, and exogenous. Mental problems were most frequently associated with Type 1 (34.9%), followed by Type 8 (21.0%), Type 2 (18.6%), and Type 3 (7.0%). Noticeable finding was that Type 1 A-bomb survivors suffered from psychogenic and exogenous mental problems in an extremely high incidence, as compared with the non-exposed group (66.3% vs 24%). The incidence of both exogenous and endogenous problems was higher in the non-exposed group (32.6% and 24.5%) than the exposed group (23.2% and 10.5%). There was no significant gender difference in the development of mental problems. According to types of A-bomb survivors, both psychogenic and exogenous mental problems were most common for Type 1. The incidence of psychogenic problems was 2.85 times higher than that of exogenous problems. (N.K.).