The risk of developing the chronic hepatitis B surface antigen (HBsAg) carrier state after primary acute hepatitis B virus infection was examined in a prospective study of adults with acute icteric (clinically apparent) viral hepatitis. Enzyme immunoassay for IgM antibody to hepatitis B core antigen (IgM anti-HBc) was positive in 176 (93.1%) and negative in 13 (6.9%) of 189 patients positive for HBsAg and negative for IgM antibody to hepatitis A virus. The former were considered to be acute hepatitis B infections and the latter chronic carriers superinfected with another virus or acute exacerbation of chronic infection. At six months, all surviving acute hepatitis B patients had cleared HBsAg. All surviving chronic carriers remained HBsAg-positive. In another prospective study, 18 adults with subclinical hepatitis B infection also cleared HBsAg within six months. These data suggest that in contrast to newborns and children, adult patients rarely develop chronic antigenemia after acute hepatitis B infection. The postulated risk should mainly be attributed to misdiagnosis of cases of superinfection of chronic carriers or acute exacerbation of chronic infection.