BACKGROUND Coinfection with human immunodeficiency virus (HIV) and hepatitis B virus (HBV) is common. Most studies have concentrated on the effects of chronic HIV infection on HBV infection; however, studies on the effects of acute HIV infection on HBV infection are especially important to elucidate the potential mechanisms leading to complications from HIV/HBV coinfection. METHODS We evaluated the HBV DNA, hepatitis B surface antigen (HBsAg), and hepatitis B "e" antigen (HBeAg) in stored serum samples from 25 men with chronic hepatitis B who had acquired acute HIV infection. RESULTS All of the 25 men had decreased HBV DNA levels during acute HIV infection. Three men converted from HBsAg positive before HIV infection to HBsAg negative during acute HIV infection, and 10 men converted from HBeAg positive before HIV infection to HBeAg negative during acute HIV infection. CONCLUSIONS These data suggest that the early effects of HIV infection improve the immune response against HBV.