The majority of studies of the A-bomb survivors in Hiroshima and Nagasaki are concerned with monitoring the populations for long-term after-effects of exposure to ionizing radiation. Nonetheless, the health surveillance programs in the two cities must also be concerned with investigation of diseases which, though not directly related to radiation exposure, are of importance to the individual's overall health. This paper describes some examples of these investigations. Surveys focussing on cardiovascular disease, diabetes, rheumatoid arthritis, thyroid disorders, gastric cancer and liver disease yield important clinical and pathological data which contribute to an understanding of the natural history of disease, benefit the health of the survivors, and make possible comparisons within Japan and with populations elsewhere in the world. Laboratory determination of levels of various serum components provide normal base line values in a healthy ambulatory population. Cytogenetic investigations yield data about normal chromosome variation, basic for future environmental pollution monitoring. Blood group gene antigen distributions, prevalences of inherited serum and red cell protein variants, and the cytogenetic information provide comparative data for a projected study of the children of the exposed generation. In the course of these population surveys, a number of extremely rare (genetic) traits unrelated to radiation exposure, such as the hereditary lack of a tissue enzyme, an unusual blood type, mutant hemoglobins, have been discovered. These traits appear to have little or no impact on the individual's health, but are of potential intrinsic interest to clinicians, geneticists, and molecular biologists. It is obvious that the voluntary participation of the A-bomb survivors in the health surveillance surveys in the two cities constitutes an outstanding contribution to the understanding of disease in man.