Mouth-to-cecum transit time was studied in 13 patients affected by chronic idiopathic constipation and 18 control subjects matched with the constipation group for age, sex, and dietary habits. In a preliminary investigation, all patients showed a prolonged whole gut (oroanal) transit time as measured with radiopaque markers. Mouth-to-cecum transit time was studied through the serial determination of breath H2 after administration of 12 g lactulose diluted in 120 ml water. Breath H2 was measured with a gas analyzer and was determined in parts per million (ppm). Breath H2 after lactulose was also determined in the group with constipation after a 10-day diet that included either glucomannan (1 g tid orally) or placebo administered in a double-blind manner. The results show a statistically significant increase in mouth-to-cecum transit time in the group with constipation, compared with controls, and a return to within the normal range after the 10-day treatment with glucomannan. With placebo, no difference in transit time was noted. We therefore suggest that chronic idiopathic constipation is a disease that involves the whole gut.