We compared lactulose syrup to a placebo (5% sucrose syrup) in 42 normal and 24 chronically constipated volunteers. The study in normal subjects was double-blind and had a crossover design of two dose levels, 30 and 60 ml/day, whereas the study in constipated subjects was a double-blind parallel study of 60 ml/day dose. Lactulose syrup produced clinically and statistically significant increases in the frequency, weight, volume, and water content of stools and produced stools of softer consistency compared to baseline values, as well as to a sucrose-treated control group in both normal and constipated subjects. The study in normals suggests that the laxative effects are dose-related. Adverse effects reported were tolerable and were extensions of the pharmacologic effects of the drug. We concluded that lactulose is an effective laxative that affects the character of the colonic fecal content, thereby inducing evacuation of feces in people with constipation.