Antecedents of family aggregation of breast and ovarian cancer are observed in only 5-8% of all breast cancer cases. Nevertheless, this variable displays one of the highest risk ratios associated to breast cancer outcome. Despite recent identification of genetic mutations associated with family aggregation of these tumors, mainly at BRCA1 and BRCA2 genes, knowledge on the interaction between environmental agents in these families remains quite unclear. In this paper we ascertained the correlation among ages of the onset of breast/ovarian cancer in 260 Brazilian families with those cancer aggregations. Further we estimated the median age of the onset of breast cancer among four generations. We observed that the higher the number of family cancer cases, the highest is the correlation of ages for the onset of breast cancer. We also observed a 8-10 year decline in the mean age-of-onset of breast/ovarian cancer from one generation to another in the studied families. If these results could be confirmed elsewhere, we believe that the hypothesis of interaction between environmental risks factors in families indeed showing breast/ovarian cancer aggregation is reinforced.