In 1968-1981, a total of 3222 serum samples were collected from healthy subjects in Okinawa--in Ishigaki City, on Hateruma Island, and on Iriomote Island--and in Kyushu, in Fukuoka City and Nichinan City. These serum samples were tested for the presence of hepatitis B surface antigen (HBsAg) by reverse passive hemagglutination (RPHA), for antibody to hepatitis B surface antigen (anti-HBs) by passive hemagglutination (PHA) and radioimmunoassay (RIA), and for antibody to hepatitis B core antigen (anti-HBc) by RIA. Overall prevalence of HBsAg (7.5%), anti-HBs by PHA (41.0%) and RIA (56.4%), and anti-HBc (65.5%) in Okinawa was significantly higher than prevalence of HBsAg (2.4%), anti-HBs by PHA (24.7%) and by RIA (28.1%), and anti-HBc (30.9%) in Kyushu. In both areas, anti-HBc was more frequently detected than anti-HBs by both methods. In Okinawa, HBsAg was significantly more prevalent in males than in females. No significant differences by sex in other hepatitis B virus markers were found. On Iriomote Island and Ishigaki City, second samples were collected after intervals of 10 and 12 years, respectively. Over these periods, the prevalence of all hepatitis B virus markers decreased significantly for the 0-9 and 10-19 year age groups. These data suggest that hepatitis B infection among children has declined in recent years and that high prevalence of hepatitis B infection in adults may reflect high rates of infection in their childhood.