A double-blind crossover trial was performed to test the therapeutic usefulness and safety of lactitol, a beta-galactoside sorbitol, against lactose in 18 patients with chronic portal-systemic encephalopathy (PSE). The study included four periods: two for washout and two for lactitol and lactose administration. During washout periods, which lasted two weeks each, patients were stabilized with neomycin plus milk of magnesia. Lactitol and lactose were administered during four weeks each. Ten patients were randomly assigned to receive lactose (group A) and eight patients to receive lactitol (group B) first. PSE parameters, ie, mental state, number connection test performance, asterixis and blood ammonia levels were assessed fortnightly. Electroencephalographic tracings and stool pHs were evaluated at the end of each study period. After the first administration of lactose and lactitol, no statistically significant differences in PSE parameters were found. At the same stage, a significant stool acidification (P less than 0.05) was detected. It is concluded that lactitol seems to be safe and efficacious in treating patients with chronic PSE.