The laxative effects of 50% lactulose syrup and 50% glucose syrup were compared in a 12-week, double-blind study of 47 elderly constipated patients living in a nursing home. The dosage was 30 ml daily taken at bedtime; it was reduced to 15 ml if the initial dosage produced two or more bowel movements daily. The number of bowel movements during treatment in comparison to pretreatment was significantly increased in the 42 patients (19 lactulose, 23 glucose) who completed at least 8 weeks of the study. Laculose was superior to glucose in the mean number of bowel movements per day (p less than 0.02) and in the percentage of days in which at least one bowel movement occurred (p less than 0.05). Reduction in the severity of each 5 symptoms (cramping, griping, flatulence, tenesmus, bloating) was greater with lactulose. For relief of all 5 symptoms, lactulose was significantly more effective than glucose (p less than 0.04). The striking reduction in the number of fecal impactions (only 6 in the lactulose patients vs 66 in the controls) was highly significant (p less than 0.015). The lactulose patients needed fewer enemas than did the controls. No abnormal values were observed in laboratory tests.