The factors that influence colonic transit time in healthy humans are not yet clearly defined. The aim of this study was therefore to determine (a) if there are differences in colonic transit time between men and women and (b) if age, female hormonal status or smoking habits are associated with alterations in these parameters. Colonic transit time was measured in 164 asymptomatic subjects (80 males, 84 females) by a radio-opaque marker technique with one single plain abdominal X-ray. Colonic transit time was significantly shorter in men than in women (30 +/- 2 vs. 42 +/- 3 h, P < 0.05). Colonic transit time in non-smoking males was significantly shorter compared with smoking males (26 +/- 2 vs. 40 +/- 5 h, P < 0.05). In females only height and menstrual cycle influenced colonic transit times. We conclude that gender and smoking habits should be considered when studying colonic transit time in health and disease.