Although prostate cancer is the most common cancer diagnosed in men in this country, we do not know why some men develop this disease and others do not. In some families, a high proportion of the men are affected with prostate cancer. In such families, the determination of whether or not a family member will develop prostate cancer is thought to result from what that person inherits from his parents. In other words, the factor which causes the prostate cancer is present in the genetic material (as a gene in the chromosomal DNA) in the sperm and/or the egg from which the individual develops. One can study prostate cancer in families to identify where in the DNA such genes are located using a technique called linkage analysis. Once the location is known, the gene itself can be identified and characterized to see how it causes the disease. This approach has been used to identify genes which cause breast cancer and colon cancer, but the genes which cause prostate cancer have not been identified, although one has been recently located on chromosome 1 (HPC1). It is the goal of the proposed research to study prostate cancer families to find and characterize additional genes which determine prostate cancer susceptibility. A series of families with multiple men affected with prostate cancer at a young age will be recruited for this study. The identification of prostate cancer genes will allow for the development of genetic tests to determine who is at high risk of developing prostate cancer, and may eventually lead to better preventative, diagnostic and therapeutic strategies to combat this common disease.