15 women complaining of chronic constipation were included in a prospective open trial to determine the laxative action and minimal effective dose of lactitol. Through the administration of increasing doses, an ED50 of 0.25 g/kg/day, was established. During a 15-day treatment period at the minimal effective dose, all patients had reduced symptoms of constipation as compared with a previous 15-day control period. Side effects of minor intensity were frequently recorded (flatulence, rumblings, wind, and, less frequently, abdominal cramps or nausea). Thus, the use of lactitol for symptomatic treatment of constipation deserves further clinical studies to determine better its indications and benefit.