Fifty-one children affected by chronic idiopathic constipation (23 males, 28 females), ranging in age from 8 months to 16 years were enrolled in the study; 42 completed the trial. The patients were divided into two groups: Group A: 19 children treated with lactitol (250-400 mg/kg/day); Group B:23 patients treated with lactulose (500-750 mg/kg/day). Parents filled a questionnaire concerning clinical response to therapy for a period of 30 days. In 17 Group A children and in 17 Group B children orocecal transit time using H2 Breath Test with lactulose was performed. A statistically significant increase of week stool frequency was found after treatment both with lactitol or lactulose (p < 0.001). Nevertheless Group B patients complained abdominal pain (p < 0.005) and flatus (p < 0.001) more frequently. Other adverse reactions, such as vomiting and meteorism, were more frequent in Group B patients (n.s.). In addition patients treated with lactitol found that sugar as more palatable and had a better compliance to the therapy. Orocecal transit time did not show statistically significant differences after the therapy with both these sugars, indicating that the activity of lactulose and lactitol occurs in the colon and that small bowel functions are not affected by a previous therapy with these sugars. In conclusion, our study demonstrate that lactitol, because of the less number of side effects compared to lactulose, should be considered as an useful agent in the treatment of chronic idiopathic constipation in childhood.