Stool weight and transit time through the gut were measured in 4 groups of 30 men, aged 50-59 years, randomly selected from populations in urban (Copenhagen) and rural (Them) Denmark and urban (Helsinki) and rural (Parikkala) Finland. These populations exhibited a 3-4 fold difference in risk for large bowel cancer. Mean transit time (37 +/- 1 hours, Copenhagen; 43 +/- 1 hours, Helsinki; 40 +/- 1 hours, Them; 37 +/- 1 hours, Parikkala) was not significantly different among populations, but average 24-hour stool weights (136 +/- 13 g, Copenhagen; 176 +/- 17 g, Helsinki; 169 +/- 16 g, Them; 196 +/- 15 g, Parikkala) were different and had a significant inverse relationship to total large bowel cancer incidence, with larger stool weights being found in the low-risk population. A high proportion of study subjects, especially in Finland, were found to be taking medication or to have a history of gastrointestinal illness, but neither of these variables related to bowel habit.