A multicentre retrospective cohort study of long-term survivors of childhood and adolescent cancer identified 7 cases of cancer among 2308 offspring (0.30%) of 2283 case-survivors and 11 cases among 4719 offspring (0.23%) of 3604 controls. Overall, the observed numbers of cases were not significantly different from those expected in the general population. Among offspring of case-survivors observed for the first 5 years of life, the group with the most person-years of follow-up, 5 cancers were reported (3 confirmed), compared with 1.7 expected, a significant excess due mostly to boys whose mothers survived cancer. Some offspring with cancer had known single-gene traits; others resembled previously recognised patterns of family cancer. The remainder may represent chance occurrences or new cancer family syndromes, such as an association with malignant melanoma. The study had an overall 79% power to detect a 3-fold excess of cancer among offspring of case-survivors, but no excess was observed. The number person-years of follow-up in the second decade of life, when most cases of cancer developed, was small.