By the end of the study, 517,000 person years in 68,000 runners (between 1991 and 2001) will be available for analysis. Survival analysis will be used to test for a dose-response relationship between running mileage and CHD and cancer risk, and whether this relationship is affected by running intensity, running frequency, running history, gender, adiposity, age or medication use. Using conservative rates (25% below published values), statistical power calculations suggest that detectable reduction in coronary heart disease risk as small as 0.71% per mile will be possible, which is far below the estimated reduction from other published studies (2.1%). Additionally, a detectable reduction in breast cancer risk as small as 1.5% per mile run in women is calculated, which is below the 1.7% reduction in risk estimated from other published data.