To investigate the efficacy and the acceptability of different doses of lactitol in patients with subclinical hepatic encephalopathy, 28 patients with cirrhosis were enrolled in a controlled clinical trial comparing 5-month therapies with lactitol at two different doses: 0.3 and 0.5 g/kg bw per day. This period was followed by 1 month of recovery. Patients were monitored with venous blood ammonia determination, three psychometric tests, clinical evaluation of mental status and EEG. The porto-systemic encephalopathy index of Conn was determined periodically. Twenty-two patients completed the trial (11 for each dose of lactitol). Both doses of lactitol decreased plasma ammonia levels and improved the porto-systemic encephalopathy index. The higher dose was more effective in improving performance in the psychometric tests. After the period of recovery, both the porto-systemic encephalopathy index and the psychometric test scores returned to pretreatment values. Lactitol was tolerated well by patients. Three patients given the higher dose reported periodic intestinal discomfort, but did not stop taking lactitol or reduce the dosage; no side-effects were reported by the patients taking the lower dose. These results indicate that lactitol in doses ranging from 0.3 to 0.5 g/kg bw is a well-tolerated and effective treatment for subclinical encephalopathy.