The treatment of HBV infection in patients with HIV co-infection presents several peculiar features: some drugs active against HBV are also active against HIV. This precludes their use in monotherapy in HIV-HBV co-infected patients due to the potential risk of selecting HIV-resistant strains. Telbivudine seemed to be a candidate for exclusive anti-HBV therapy because it exerts no significant in vitro activity against HIV. In this context, we describe the case of a HIV-HBV co-infected patient who presented indication for treatment only for HBV infection. After a short course of interferon treatment withdrawn due to adverse events, adefovir monotherapy was started. Since no significant viral drop was achieved during adefovir treatment, telbivudine was added. This treatment was associated with a complete virological response on HBV. It is noteworthy that after two months of this treatment even the HIV viral load presented a significant reduction. Our findings pose concerns of possible antiviral activity of telbivudine against HIV and therefore of selecting resistant mutations.