Familial cancer in the general population.

In the course of collecting samples of human milk for the National Cancer Institute, family histories have been assembled since 1971. Ages, causes of death, reproductive histories, and the history and age of occurrence of cancer were obtained for all first degree relatives of the proband's parents. Two thousand and ninety cancers were identified in 31,945 persons. Cancers were confirmed by pathology reports, hospitals' and doctors' records, or death certificates. Of the family lineages, 65.9% had no cancers and 8.0% had two or more. For all cancers taken together, clustering was not significant but there was significantly more observed than expected cancers in lineages with cancer of breast, ovary, skin, corpus uteri, stomach, rectum, lung and bronchus, or colon. Little or no excess was observed in lineages with leukemia, lymphoma, and cancer of the cervix or prostate. The relative risk of breast cancer was increased 1.5 times for daughters and 3.8 times for sisters of women with the disease.