Mild to moderate fat malabsorption is frequently present in patients with liver cirrhosis. We investigated the influence of lactulose or lactitol treatment on fecal fat excretion in 18 patients with liver cirrhosis. All patients were Child Pugh class A or B and had not taken any therapy that could have affected intestinal absorption in the previous months. The dose of lactulose or lactitol was individually adjusted to maintain two semiliquid bowel movements per day. Steatorrhea was determined before and after a minimum of 7 days, when the cathartic effect was stabilized. Treatment with nonabsorbable disaccharides induced mild to moderate steatorrhea in 50% of patients. No differences were observed between the effects of lactulose and lactitol, but fecal fat excretion exceeded 10 g/day in two patients taking lactulose. These findings indicate that treatment with nonabsorbable disaccharides may increase fecal fat excretion in patients with liver cirrhosis. This factor should be taken into consideration when a cirrhotic patient has to take these drugs for a long time.