To assess the effects of atomic bomb radiation on the incidence of multiple primary cancers (MPC), we analyzed the association between the incidence of second primary cancers in survivors of the atomic bombing of Nagasaki, and exposure distance. The incidence rate (IR) of a second primary cancer was calculated and stratified by the distance from the hypocenter and age at the time of bombing for the years 1968 through 1999. The IR of the first primary cancer was also calculated and compared with the second primary cancer to determine whether atomic bomb radiation was associated with the multiplicity of tumors. There were 511 confirmed cases of MPC in the 7572 cancer-bearing survivors. The crude IR was 27.6 per 100,000 person-years. The IR of second primaries decreased significantly with increasing distance from the hypocenter: relative risk, 0.89 per 1.0 km; 95% confidence interval, 0.84-0.94. A significant decrease was also noted for those of older ages at the time of the bombing, based on the attained age of the second primary cancer: relative risk, 0.91 per 1 year; 95% confidence interval, 0.90-0.92. These findings suggest that the radiation has affected the incidence of MPC. Furthermore, when compared with the first primary cancer, a stronger distance effect was suggested on the occurrence of a second primary cancer in the survivors. The present study suggests the significance of atomic bomb radiation on MPC in the survivors. These results on the incidence of MPC in the tumor-bearing survivors and its correlations with the atomic bombing of 62 years ago are described for the first time in this report.