Recently, entecavir was introduced as a potent drug against hepatitis B virus infection. Initially, it was suggested not to have any effect on human immunodeficiency virus (HIV) infection. This guideline was revised in 2007 because of a report showing that the M184V mutation was selected in an hepatitis B virus and HIV-coinfected patient previously treated with lamivudine. Our investigation revealed findings similar to those preveiously reported but in an antiretroviral therapy-naive patient coinfected with HIV and hepatitis B virus. After 26 weeks of entecavir therapy, the M184V mutation dominated the plasma viral population. Thus, entecavir should only be used for coinfected patients who simultaneously receive suppressive therapy against HIV infection.