We have provided intensive medical-genetic follow-up on a previously published cohort of 225 consecutively ascertained patients with verified breast cancer evaluated in our oncology clinic. Similar evaluation was performed on 103 newly ascertained breast cancer patients, giving a total of 328 consecutive patients with verified breast cancer. We believe that this represents one of the most intensive efforts of its type for meticulous documentation of genealogy and cancer of all anatomic sites on a series of breast cancer patients. The results showed that the familial and putative hereditary categories almost doubled among our original 225 breast cancer probands. We conclude that the impact of the host factor component in breast cancer etiology, as reflected in most reports in the literature, suffers from severe underestimation.