Our retrospective cohort study found that men diagnosed with breast cancer have a 16% higher risk of suffering a second cancer (not a spread of the original tumor) than the general male population. Data analyzed from the California Cancer Registry during the period 1988-2003 included 1,926 men aged 85 or younger diagnosed with a first primary breast cancer. These men are especially prone to developing a second primary breast, stomach and melanoma skin cancer. One possible explanation for the association between the first and second cancer include genetic mutations in the BRCA1 and BRCA2 genes. Male tumors related to BRCA1 and BRCA2 include breast, melanoma, stomach, prostate, colon and pancreatic cancer. Further, the particularly high risk for bilateral breast disease and the high risk among early onset cases are an indicator of a hereditary cancer syndrome. Other possible explanations for the associations include shared hormonal abnormalities, and shared lifestyle and environmental risk factors between the first and second cancer. Studying the occurrence of second primary cancers holds important implications for cancer etiology and preventive medicine. These results will have substantial medical implications for cancer patients as well as for relatives that are at risk of the disease. Medical interventions to address increased risk of second cancer include increased surveillance, chemoprevention, and prophylactic surgery.