We compared the effect of lactulose or lactitol on the fecal flora of 21 cirrhotic patients without hepatic encephalopathy. All were treated with an individualized disaccharide dose to achieve and maintain two semiliquid bowel movements per day. Stool pH and fecal flora were determined before and 10 days after stabilizing the cathartic effect. Increased counts of lactobacilli were obtained with both treatments. This increase, which was related to the decreased stool pH, was more constant with lactulose. In addition, lactitol decreased certain proteolytic bacteria such as enterococci and enterobacteria. Both total aerobic and anaerobic bacterial counts showed little quantitative variations with either treatment.