In a double-blind, randomized study the efficacy of lactulose was compared with neomycin-sorbitol in 45 episodes of acute nitrogenous portal-systemic encephalopathy (PSE) induced by dietary protein, azotemia, or gastrointestinal hemorrhage. All patients had underlying cirrhosis, and at the time of randomization had encephalopathy of at least grade 2 severity and arterial ammonia concentrations greater than 150 microgram/100 ml. Two thirds of the patients in each group returned to normal mental status and more than 80% in each group showed at least one grade improvement in mental state. In addition, there was equivalent improvement in asterixis, in the performance of the Number Connection Test, in the electroencephalographic pattern, and in arterial ammonia concentration. The principal difference between the two groups was a greater reduction in stool pH after lactulose therapy than after neomycin-sorbitol therapy. One patient randomized to neomycin-sorbitol had to be withdrawn from the study because of persistent vomiting related to the administration of the medication. Otherwise there were no complications attributable to therapy in either group. These data suggest that neomycin-sorbitol and lactulose are equally effective in the treatment of acute nitrogenous portal-systemic encephalopathy.