A 45-year-old male active homosexual was given a diagnosis of HIV-1 and acute hepatitis B in August 2007. Since his liver function became rapidly impaired, anti-HBV therapy with oral administration of entecavir (ETV) was started, and resulted in a favorable outcome. However, serum concentration of HIV-RNA decreased by log 1.26 within 60 days, which strongly suggested the inhibition of HIV proliferation by ETV. To prevent the appearance of mutated HIV, novel therapeutic strategies should be established in HIV/HBV-coinfected patients.