Before the start of the study, 233,000 person-years of follow-up had been accumulated in 56,000 runners (between 1991 and 1997). The runners were resurveyed in 1997 along with 68,000 walkers. The walkers were also solicited through the publication of the questionnaire in Walking magazine followed by a direct mailing of the questionnaire to 425,000 subscribers. Total and cause-specific mortality will be determined from the National Death Index; fatal and nonfatal cancers will be identified from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) and 46 state registries; nonfatal coronary heart disease and injuries will be determined from questionnaires. Survival analyses will be used to test whether runners have greater reduction in heart disease, total mortality, and cancer per unit of exercise. Exercise-related injuries from walking and running will also be examined. Power calculations suggest that detection of differences between runners and walkers, as small as 11% for total mortality, 16% for CHD, 12% for total cancers, and 36% for breast cancer, will be possible. The differences will be adjusted for weekly kilocalories expended by walking and running, for walking and running distance, and for time spent on each activity to test whether these variables account for differences in disease rates between walkers and runners.