A 57-year-old man developed acute hepatitis B virus (AHB), caused by HBV genotype Ae. Lamivudine (LAM) therapy was started at 8 months after the disease onset, because the infection was persistent, but not self-limited. Despite LAM therapy, the hepatitis became chronic. Further, virological breakthrough developed due to the emergence of LAM-resistant YMDD mutants at 11 months after LAM therapy. Adefovir dipivoxil (ADV) was combined with LAM against breakthrough hepatitis at 28 months after LAM therapy. Sequential genetic analysis revealed that rtL217R, a mutation potentially diminishing the ADV efficacy, was detected before and after the combination therapy. During the follow-up period, the patient unexpectedly turned out coinfected with human immunodeficiency virus (HIV) by measuring anti-HIV-1 antibody. At that time, LAM-resistant HIV mutation, M184V, had been already detected. We switched from the combination therapy with LAM plus ADV to highly active antiretroviral therapy (HAART), which included tenofovir disoproxil fumarate. HAART drastically improved LAM-resistant viremia and breakthrough hepatitis as well as HIV viremia and CD4 counts. Even in Japan, HBV genotype and HIV coinfection should be determined early in the treatment of AHB, and early induction of nucleotide analogs should be taken into consideration, because the proportion of AHB patients with HBV genotype A and the number of patients horizontally coinfected with HBV and HIV are increasing.