Breast cancer seems to represent a heterogeneous group of neoplasia originating from the parenchymal epithelium of the mammary gland. Family studies combined with genetic epidemiological analyses and histological evaluations were used to gain an insight into this nosological entity. Special emphasis was given to the type and frequency of neoplasia in close relatives. This study represents pedigrees of 36 histologically defined pure tubular and 22 pure medullary breast cancer patients as well as 171 with the invasive ductal form. The incidence of cancer in the first degree relatives of all three groups is compared to that of the local population. The first degree relatives did not have a higher risk (RR) for the neoplastic diseases. However, breast cancer occurs more frequently in the female relatives of all three groups. Other cancers have different relative risks.