Cancer treatments have the potential to cause germline mutations that might increase the risk of cancer in the offspring of former cancer patients. This risk was evaluated in a population-based study of early onset cancer patients in Finland. Using the nationwide registry data, 26,331 children of pediatric and early onset cancer patients (diagnosed under age 35 between 1953 and 2004) were compared to 58,155 children of siblings. Cancer occurrence among the children was determined by linkage with the cancer registry, and the standardized incidence ratios (SIRs) were calculated comparing the observed number of cancers with that expected, based on rates in the general population of Finland. Among the 9,877 children born after their parent's diagnosis, cancer risk was increased (SIR 1.67; 95% CI 1.29-2.12). However, after removing those with hereditary cancer syndromes, this increase disappeared (SIR 1.03; 95% CI 0.74-1.40). The overall risk of cancer among the offspring of siblings (SIR 1.07; 95% CI 0.94-1.21) was the same as among the offspring of the patients with non hereditary cancer. Risk of cancer in offspring, born before their parents cancer diagnosis, was elevated (SIR 1.37, 95% CI 1.20-1.54), but removing hereditary syndromes resulted in a diminished and nonsignificant association (SIR 1.08, 95% CI 0.93-1.25). This study shows that offspring of cancer patients are not at an increased risk of cancer except when the patient has a cancer-predisposing syndrome. These findings are directly relevant to counseling cancer survivors with regard to family planning.