Sorbitol solution has been used to a control substance of evaluating the efficacy of lactulose therapy for hepatic encephalopathy (HE). However, recent in vitro studies suggested that sorbitol may be an inert placebo and may have therapeutic relevance. We evaluated in vivo metabolism of sorbitol in healthy volunteers and alcoholic cirrhotic patients and found that sorbitol was metabolized by gut bacteria in a similar manner to lactulose. We next evaluated the effect of sorbitol treatment on five psychomotor performances tests in cirrhotic patients. Patients receiving sorbitol demonstrated improvement in all psychomotor tests, whereas similar patients not receiving sorbitol showed no improvement. We conclude the following: sorbitol is metabolized by gut bacteria in man, sorbitol therapy improved psychomotor performance in cirrhotic patients, and previous studies using sorbitol as a control underestimated the beneficial effects of lactulose.