To evaluate the effects of continuous low-level ionizing radiation on humans, the follow-up data (1980-85) on Japanese thorotrast-exposed patients were analyzed. The patients were 241 war-wounded military personnel registered with and cared for by the Ministry of Health and Welfare since 1979. During this period, a total of 1144 person-years, 94 patients died. Compared with the expected number of deaths calculated from age- and cause-specific death rates in Japan during the same period, the thorotrast-exposed patients were at three times greater risk of death from all causes (P less than 0.001), had 47 times the risk of liver cancer (P less than 0.001), 12 times the risk of leukemia (P less than 0.05), and 20 times the risk of liver cirrhosis (P less than 0.001). Age at time of thorotrast injection, drinking and smoking habits had little effect on these statistics. Analyses of 30 autopsied patients with liver cancer showed statistically significantly increases in hemangiosarcoma and cholangiocarcinoma. The thorotrast-exposed patients' estimated risk of liver cancer by histological type was 21 times that of the general population for hepatocellular carcinoma, 303 times that for cholangiocarcinoma and 3129 times that for hemangiosarcoma.