As part of a multi-centre epidemiological study of cancer in women between the ages of 20 and 54, data were collected concerning family history of gynaecological cancers in the female relatives of 4730 women with newly diagnosed breast cancer and the relatives of 4688 women from the general population. Women who were diagnosed with breast cancer prior to age 45 were more likely than controls to have a mother or sister with ovarian cancer (odds ratio (OR): 1.50), endometrial cancer (1.29), and cervical cancer (1.53), although none of these elevations achieved statistical significance. The corresponding odds ratios for women diagnosed with breast cancer between the ages of 45 and 54 were 1.88, 0.84 and 0.93. The association with ovarian cancer was statistically significant in this group (95% confidence interval (CI): 1.11-3.19). In this latter group, having a first degree relative with ovarian cancer was associated approximately as strongly with breast cancer as was having a first degree relative with breast cancer. The results suggest that there may be a shared genetic basis for some cancers of the breast and ovary. From a clinical perspective, the results indicate that in setting appropriate levels of screening for breast cancer and in establishing an appropriate age at which to begin such screening for a particular woman, her family history of ovarian cancer should be considered in addition to her family history of breast cancer.