Entecavir is a potent nucleoside inhibitor of the hepatitis B virus (HBV) polymerase with a high antiviral efficacy and a high genetic barrier to viral resistance. After approval in 2006, knowledge on the side effect profile in patients with advanced liver disease and impaired liver function is still limited. Here, we report on 16 patients with liver cirrhosis and chronic hepatitis B who were treated with entecavir. Five of these patients developed lactic acidosis during entecavir treatment. All patients who developed lactic acidosis had highly impaired liver function (Model for End-Stage Liver Disease [MELD] score >or= 20). Lactic acidosis (lactate 26-200 mg/dL, pH 7.02-7.40, base excess -5 mmol/L to -18 mmol/L) occurred between 4 and 240 days after treatment initiation with entecavir. Lactic acidosis was lethal in one patient but resolved in the other cases after termination/interruption of entecavir treatment. No increased lactate serum concentrations were observed during treatment with entecavir in the other 11 patients with chronic hepatitis B and liver cirrhosis who all had MELD scores below 18. The MELD score correlated with the development of lactic acidosis (P < 0.005) as well as its single parameters bilirubin, international normalized ratio, and creatinine. In contrast, Child-Pugh Score did not correlate with the development of lactic acidosis. Our data indicate that entecavir should be applied cautiously in patients with impaired liver function.