We have examined whether parental exposure to atomic bomb radiation has led to increased cancer risks among the offspring. We studied 40,487 subjects born from May 1946 through December 1984 who were cancer-free in January 1958. One or both parents were in Hiroshima or Nagasaki at the time of the bombing and for childbirth. Using population-based tumor registry data we analyzed cancer incidence data from 1958 to 1997 by Cox regression models, and we examined the effects of both paternal and maternal irradiation with adjustment for city, sex, birth year, and migration. During follow-up, 575 solid tumor cases and 68 hematopoietic tumor cases were diagnosed. Median age at diagnosis was 39.7 years. Median doses were 143 millisierverts for 15,992 exposed (5+ millisierverts or unknown dose) fathers and 133 millisierverts for 10,066 exposed mothers. Cancer incidence was no higher for subjects with exposed parents than for the reference subjects (0-4 millisierverts), nor did the incidence rates increase with increasing dose. For 3568 subjects with two exposed parents, the adjusted risk ratio for all cancer was 0.97 (95% confidence interval 0.70-1.36). Because of the small number of cases, however, we cannot exclude an increase in cancer incidence at this time.