The influence of age, origin, and region of domicile on the prevalence of hepatitis B virus markers was investigated in a cross-sectional sample of young Swedish adults. Sera and demographic data were prospectively collected from 2,000 male conscripts and 2,000 pregnant women from urban and rural parts of Sweden during 1988-1989. A total of 10 of 4,000 (0.25%) were found to be hepatitis B surface antigen carriers, and 62 (1.6%) were positive for both anti-hepatitis B core antigen and anti-hepatitis B surface antigen, indicating a present or prior hepatitis B virus infection. Origin from a country with high hepatitis B virus endemicity and increasing age were factors associated with serologic signs of a present or prior hepatitis B virus infection, whereas region of domicile was not. In addition, seven individuals positive only for anti-hepatitis B core antigen (blocking greater than 70% in the test) and 39 positive only for anti-hepatitis B surface antigen (with a titer of greater than or equal to 10 international units/liter) were believed to have been exposed to hepatitis B virus. Altogether, these results indicated that 3% of young Swedes had encountered hepatitis B virus and that origin from a country with high hepatitis B virus endemicity was the factor that most strongly was associated with a present or past hepatitis B virus infection.