Family studies of prostate cancer, in keeping with studies of many other common cancers, have shown an increased risk to relatives of cases over and above the general population. Male relatives of early onset prostate cancer cases are also at increased risk over relatives of later onset cases. These observations, together with a few families with four or more cases of prostate cancer, suggest that there is a high penetrance, inherited form of this cancer. Further evidence comes from the increased risk of prostate cancer in BRCA1 mutation carriers but more particularly from the recent report of the mapping of a prostate cancer susceptibility gene to chromosome 1.