Regarding the limited evidence for determining the optimal duration of antiviral treatment for hepatitis B, the long-term outcome of patients with favorable responses to over 5 years of lamivudine monotherapy was investigated. Two hundred seventy-one patients who had received lamivudine for at least 5 years were enrolled. Ultimately, 72 patients without YMDD mutations and showing hepatitis B virus (HBV) DNA levels <2.5 pg/ml after 5 years of treatment were analyzed. Mean treatment duration with lamivudine was 9.1 ± 2.6 years. During the treatment, HBeAg and HBsAg loss/seroconversion rates were 95 and 6.9%, respectively. Decompensation and hepatocellular carcinoma (HCC) developed in 2.8 and 6.9% of patients, respectively. Old age and cirrhosis were risk factors for HCC development. Finally, 11.1% of patients developed YMDD mutations after 8.3 ± 2.4 years of treatment. There was no hepatic decompensation among the patients who developed delayed YMDD mutations. Sixteen patients who achieved a complete response stopped lamivudine and four patients showed relapses 10.3 ± 8.5 months after stopping lamivudine. Relapsed patients had more cirrhotic livers and higher rates of HBeAg positivity at 5 years than patients who maintained complete response. The present study suggests that patients who do not develop YMDD mutations over 5 years of treatment with lamivudine may continue lamivudine monotherapy until the loss of HBsAg. However, even for the patients showing favorable response over 5 years of treatment, those in older ages, with cirrhosis or who show poor HBeAg responses should be on careful monitoring to detect the development of viral mutations, relapse and even HCC. J. Med. Virol. 85:34–42, 2012. © 2012 Wiley Periodicals, Inc.