The application proposed to continue and extend the analysis of blood samples that were collected at baseline as part of the Physicians' Health Study (PHS), a randomized trial of aspirin and beta-carotene among 22,071 U.S. male physicians, aged 40-84 years. Blood samples were collected prior to randomization, mostly in 1982, from 14,916 participants, free from prior myocardial infarction (MI), stroke, or cancer (excluding non- melanoma skin cancer), and were stored at -82C. Using a nested case- control design, the plan is to continue to analyze the specimens for nutritional and biochemical/genetic markers of risk, with a specific focus on prostate and colorectal cancers. The studies are directed toward exploring the hypothesis that dietary and genetic factors, which increase intracellular folate, reduce risk of colon cancer. Measurements of biochemical markers (plasma, folate, vitamins B6 and B12), genetic polymorphisms for key enzymes, and dietary variables will be carried out. For both prostate and colorectal cancer, vitamin D levels and metabolism (including polymorphisms of the vitamin D receptor) will be assessed. Additional experiments will test the hypotheses that high levels of insulin-like growth factor (IGF) and low levels of selenium are associated with risk of colorectal and prostate cancer. These analyses are based in the PHS, for which follow-up of disease endpoints is already supported. Furthermore, there was a long period of beta-carotene supplementation for half the cohort, providing the opportunity to assess several interactions. The accrual of cases over a long duration since the blood was drawn will permit an assessment of latency. The completeness of follow-up and the many assays already performed can be used to complement any further studies. Several important and intriguing results have already emerged from the study. The nested case-control approach and the multiple analyses have provided an efficient means to increase the scientific value of PHS and test a number of hypotheses of potential public health importance, using blood samples that were collected at considerable expense and stored with great care.