Morphologic type is not generally included in the study of cancer risk factors. In breast cancer, attention is being given to age at diagnosis. In this study cases were analyzed by morphologic type as well as age at diagnosis for occurrence of bilateral disease and for family history of breast cancer. Of the morphologic types which were more frequent in younger patients--lobular in situ, medullary, and intraductal comedo--only lobular in situ had an increased frequency of bilaterality (fourfold increase). Of these, only patients with intraductal comedo reported familial breast cancer more frequently than the average. Lobular infiltrating carcinoma is diagnosed more frequently in elderly patients and is associated with 2.6 times more bilaterality and increased familial risk. Diagnosis at age 45 to 54 is also associated with increased bilaterality, but the diagnosis of lobular carcinoma has much stronger association with bilaterality than early age at diagnosis. Because of the high risk of bilaterality in lobular neoplasia and in those with a family history of cancer occurring before the menopause, bilateral biopsies and subsequent screening for malignancy should be done in these women.