Fifty-eight patients, 30 males and 28 females, aged between 42 and 60 years (mean age 57 years) suffering from cirrhosis of the liver with signs and symptoms of portosystemic encephalopathy were studied. The double-blind study was performed according to a double-dummy experimental design, comparing the antibiotic rifaximin, administered at a dose of 1200 mg/day, with lactulose, administered at a dose of 30 g/day, both for 15 days. At the end of the treatment with the two drugs, there was a significant improvement in the main symptoms of portosystemic encephalopathy (mental state, asterixis, 'A' cancellation test, Reitan test). The improvement, which was also confirmed by a reduction in the overall score attributed to the degree of portosystemic encephalopathy was undoubtedly correlated with the reduction in the levels of serum ammonia concentrations, recorded after only 3 days of treatment. Tolerability of the treatment with rifaximin was decidedly higher with respect to lactulose. The greater rapidity of action and the lack of side-effects with rifaximin recommend the use of this intestinal antibiotic in patients with medium to severe portosystemic encephalopathy.