This proposal focuses on incident hepatitis B virus (HBV) infections in men in the Multicenter AIDS Cohort Study (MACS), which is a study of men who have sex with men (MSM). In particular, the goals include determining trends in incident HBV infections in HIV-infected and HIV-uninfected men during the pre-HAART and HAART eras and determining factors associated with various outcomes of these infections. These goals are important to achieve since liver disease is an important cause of morbidity and mortality in HIV-infected persons and these data will provide information to focus public health efforts on prevention of HBV transmission. The first Aim determines the incidence of new HBV infections during distinct 5 year periods that cover the pre- HAART, early HAART, and later HAART periods in order to determine how the incidence has changed over time in HIV-infected and HIV-uninfected MSM. Other sexually transmitted diseases have been increasing in this population since HAART has become more effective but data on HBV incidence do not exist. These data are important for public health efforts in this population. The second Aim will determine whether transmission of drug-resistant HBV is occurring. The MACS is an ideal population to study this since many of the HIV- infected men are on HIV drugs that are also active against HBV, which is the group most likely to be infected with a drug-resistant variant. The third Aim is designed to determine factors associated with recovery from an acute HBV infection in the pre-HAART and the HAART eras. The fourth Aim will determine the frequency of seroreversion and the factors associated with seroreversion. In summary, this study will yield important information to understand the current HBV epidemic amongst MSM in order to target public health efforts more efficiently.