Saudi Arabia is considered to be an area of endemic hepatitis B virus (HBV) infection. By adult age, 7% persons have hepatitis B surface antigen (HBsAg) and about 70% have one or more HBV markers. In order to provide a baseline for the integration of hepatitis B vaccine into the extended programme of immunisation (EPI), a population-based survey of HBV markers was made among Saudi children. The overall prevalence of HBsAg was 6.7%, with at least one HBV marker being positive in 19.7% persons tested. Two peaks of HBV prevalence were observed in the 7- and 10-year-old children respectively. The prevalence of HBsAg was steady in all age groups with identifiable but insignificant peaks in children aged 4 and 7 years respectively. Despite the apparent homogeneity of the Saudi population, the prevalence rates of HBV varied among the regions and were higher in urban dwellers. There was no significant difference in the HBsAg prevalence for the sexes (7.3% for males and 6.0% for females). Socioeconomic factors and family size did not significantly influence the prevalence of HBV among children. Of 307 HBsAg-positive children, 55 (17.9%) were positive for HBeAg. The early acquisition of HBV in the Saudi population is confirmed. The most effective strategy for HBV control, therefore, is by mass vaccination of all Saudi infants. An extension of the immunisation programme so as to include all pre-school children should further reduce the reservoir of HBV in Saudi Arabia. A repetition of a similar survey after 5 and 10 years should be made in order to measure this reduction.