OBJECTIVE Minimal hepatic encephalopathy (MHE) in patients with liver cirrhosis may have prognostic significance with regard to the development of clinical hepatic encephalopathy (HE) and deterioration in patient quality of life. Its prevalence in acute viral hepatitis (AVH) is not known. METHODS Consecutive 20 AVH patients (age, 29.9+/-7.9 years; M:F 18:2, hepatitis A:B:E: 2:16:2) without overt encephalopathy were evaluated for MHE and followed up. All patients underwent number connection tests - A and B, figure connection tests - A and B, digit symbol test and object assembly test and critical flicker frequency (CFF) at baseline and after the resolution of icterus. MHE was diagnosed if two or more psychometric tests were abnormal. RESULTS Prevalence of MHE (n=5) was 25%, which resolved on follow-up during the anicteric resolution phase. Five (25%) patients had greater than two abnormal psychometry tests and four (20%) had CFF <38 Hz. CFF alone had sensitivity and specificity of 80 and 100%, respectively, in the diagnosis of MHE. There was significant difference in the performance of CFF during the icteric and resolution phase of AVH (40.6+/-3.4 vs 41.8+/-2.1 Hz, P=0.04). Arterial ammonia level were higher in patients with MHE compared with patients without MHE (88.2+/-23.5 vs 53.8+/-10.9 micromol/L, P=0.001). On univariate analysis fasting ammonia level at baseline was significantly associated with all the psychometric tests (P=0.001). None of the patients developed HE either in MHE group or in those who did not had MHE at baseline. CONCLUSIONS MHE occurs in 25% of patients with AVH and resolves on follow up with recovery of AVH. Raised arterial ammonia during the icteric phase is associated with development of MHE.