Background: Hepatic encephalopathy is a major complication of cirrhosis and is associated with a poor prognosis. Objective: To identify mutations in the gene sequence for glutaminase in humans that could be responsible for the development of hepatic encephalopathy in patients with cirrhosis. Design: Cohort study. Setting: Outpatient clinics in 6 Spanish hospitals. Patients: 109 consecutive patients with cirrhosis in the estimation cohort, 177 patients in the validation cohort, and 107 healthy control participants. Measurements: Patients were followed every 3 or 6 months until the development of hepatic encephalopathy or liver transplantation, death, or the end of the study. Results: The genetic analyses showed that glutaminase TACC and CACC haplotypes were linked to the risk for overt hepatic encephalopathy. Mutation scanning of the glutaminase gene identified a section in the promoter region where base pairs were repeated (a microsatellite). Over a mean follow-up of 29.6 months, hepatic encephalopathy occurred in 28 patients (25.7%) in the estimation cohort. Multivariable Cox models were used to determine the following independent predictors: Child-Turcotte-Pugh stage (hazard ratio [HR], 1.6 [95% CI, 1.29 to 1.98]; P = 0.001), minimal hepatic encephalopathy (HR, 3.17 [CI, 1.42 to 7.09]; P = 0.006), and having 2 long alleles of the microsatellite (HR, 3.12 [CI, 1.39 to 7.02]; P = 0.006). The association between 2 long alleles of the microsatellite and overt hepatic encephalopathy was confirmed in a validation cohort (HR, 2.1 [CI, 1.17 to 3.79]; P = 0.012). Functional studies showed higher luciferase activity in cells transfected with the long form of the microsatellite, which suggests that the long microsatellite enhances glutaminase transcriptional activity. Limitation: Other genes and allelic variants might be involved in the clinical expression of hepatic encephalopathy. Conclusion: This study identifies a genetic factor that is associated with development of hepatic encephalopathy in patients with cirrhosis. Primary Funding Source: Instituto de Salud Carlos III, Spanish Ministry of Health. © 2010 American College of Physicians.