During a 65-month period, 2,935 adults (mean age, 37 years) kept computer logs of amount and intensity of exercise performed. A total of 374,798 person-hours of exercise, including 2,726,272 km or running and walking, was recorded. Two cardiac events and no deaths occurred during this period. Considered in age-specific categories, the maximum risk estimates (MREs) consistent with these data (upper 95% confidence limits) range from 0.3 to 2.7 events per 10,000 person-hours of exercise for men and 0.6 to 6.0 events per 10,000 person-hours for women. The MREs for women were higher because there were fewer women in the study. If exercise were performed three time per week for 30 minutes per session for a year, these results would lead to MREs from .002 to .027 events per person-year for men and .005 to .05 events per person-year for women. Actual risks are likely to be lower.