One hundred twenty-six patients with cirrhosis, hyperammonemia (>50 micromol/L), and chronic (persistent) hepatic encephalopathy (HE), which developed spontaneously without the existence of known precipitating factors, were enrolled in a randomized, double-blind, placebo-controlled clinical trial of intravenously administered L-ornithine-L-aspartate (OA). Patients with subclinical (grade 0, West-Haven criteria) hepatic encephalopathy (SHE), characterized by a prolonged number connection test A (NCT-A) time, and manifest HE (grades I and II, West-Haven criteria) were included in the investigation. The trial was planned as a confirmatory clinical trial OA administered in a dose of 20 g/d, as well as placebo, were dissolved in 250 mL of 5% fructose and infused intravenously for a period of 4 hours during 7 consecutive days with a superimposed protein load at the end of the daily treatment period. Primary variables were postprandial venous ammonia and NCT-A performance time measured following OA or placebo infusions to evaluate the net effect of the treatment on the prevention of the protein-induced hyperammonemia, and on parameters such as NCT-A influenced by hyperammonemia. Mental state gradation, portal systemic encephalopathy index (PSEI), and fasting ammonia levels were estimated as additional efficacy parameters. The data presented are based on the total study sample (intent-to-treat analysis), which included 63 patients in the placebo group and 63 patients in the OA group. Of the 126 patients, 114 met all the criteria for inclusion and completed the trial and treatment as outlined in the protocol (treated-per-protocol analysis). During baseline, the placebo and treatment groups were homogeneous with regard to mental states, NCT-A performance time, fasting venous blood ammonia levels, and Child-Pugh criteria. Although a slight improvement occurred in the placebo group, NCT-A performance times (P < .001) and postprandial venous ammonia concentrations in the OA-treated group showed improvements in comparison with placebo. In addition, venous fasting blood ammonia concentration (P < .01), mental state gradation (P < .001), and PSEI (P < .01), which includes the mental state gradation, NCT-A time, and postprandial venous ammonia in this trial, improved to a much higher degree in the OA group than in the placebo group. In subgroups retrospectively classified according to their initial mental state gradation, OA showed differential but uniformly significant efficacies in patients with manifest HE with respect to ammonia-lowering, improvement in NCT times, and mental state gradation. In patients with initial SHE, OA revealed differences between the medications in the psychometric test used. Adverse events consisting of mild gastrointestinal disturbances were observed in 3 of the OA-treated patients (5%). OA infusion appears to be a safe, effective treatment of chronic (persistent) manifest HE in cirrhotic patients. Additional investigations are required to assess the efficacy of OA in patients with SHE, as well as in patients with more severe grades of HE.