To quantify hereditary factors in the risk of cancer, we matched 1283047 cancer patients listed in the Swedish Cancer Registry with healthy controls from a national database, and identified the number of individuals who were first-degree relatives of other individuals in the same cohort. Division of the number of relatives in the patient cohort by the number in the control cohort yielded a "familial index". The following cancers had high familial indices: eye 16.5 (95% CI 2.5-666), testis 9.0 (3.2-35), Hodgkin's disease 6.5 (2.3-26.0), and thyroid 6.2 (3.7-12). Overall, however, familial factors made only a minor contribution to susceptibility to cancer.