The relationship between a family history of cancer and the risk of breast cancer was investigated in a study of 495 breast cancer cases and 785 controls aged 20 to 56 years. A positive association was found between the occurrence of breast cancer and a history of breast cancer in families. This relationship increased linearly with both the number of the affected relatives and with the degree of kinship between the affected relative and the case. The highest risk was observed when a sister was affected by breast cancer. This could be explained by longer common environmental exposures between sisters than between mother and daughter. This could also be explained by a genetic factor segregating under a recessive model. The risk of breast cancer associated with colon, uterus, ovary, and prostate cancer in the family was not significantly different from one. However, the estimated odds ratios associated with a family history of colon cancer increased with the degree of kinship between the affected relative and the case in a similar manner to those of breast cancer. A relationship between the risk of breast cancer and a family history of colon cancer would support the existence of a common familial factor (be it genetic or not) for these cancers. Further genetic epidemiological studies might help to define the mode of inheritance of the same susceptibility to cancer at different sites.