A population-based case-control study was conducted to assess the relation between physical activity and colon cancer among men and women aged 30-62 years. Cases were 251 men and 193 women diagnosed with colon cancer in 1985-1989 in three countries in the Seattle metropolitan area who were identified from the Seattle-Puget Sound Surveillance, Epidemiology, and End Results registry. Controls were 233 men 194 women identified by random digit telephone dialing who were selected by stratified random sampling to approximate the age, sex, and county distribution of cases. Physical activity was assessed by questions on frequency and duration of types of recreational and occupational activities during the 10-year period ending 2 years before diagnosis. Each activity was classified as low intensity (< 4.5 METs ) or moderate to high intensity (> or = 4.5 METs). For men and women combined, moderate or high intensity recreational activity was associated with a decreased risk of colon cancer (relative risk (RR) for two or more times per week vs. none = 0.70, 95% confidence interval (CI) 0.49-1.00). This relation was stronger for men than women. Occupational activity was not associated with colon cancer, except among men younger than 55 (RR for > or = 14.5 hours per week of moderate activity vs. none = 0.29, 95% CI 0.12-0.69). Among men and women combined, total moderate or high intensity activity (occupational plus recreational) was marginally related to colon cancer (RR for > or = 5 hours per week vs. none = 0.78, 95% CI 0.55-1.10). These results were adjusted for age (and sex in the combined sex analyses) and were not confounded by body mass index, dietary factors, or other measured health behaviors. The results of this study provide modest support to the growing number of studies showing that recreational and/or occupational physical activity is associated with a reduced risk of colon cancer.