A seroepidemiological survey of a sample – roughly representative by age and sex – of 2744 persons of the Greater Athens area revealed that hepatitis B virus (HBV) infections are highly endemic in Greece. Hepatitis B surface antigen (HBsAg) was detected in 72 (2·6 %) of them. The subtype was identified in 70 of the 72 carriers, and 69 were ay; the other was ad. Determinant w was present in all 61 that were capable of being typed further, and a23y(w) was twice as common as a11y(w). Antibody to HBsAg (anti-HBs) was found in 514 (18·7%) persons. The prevalence of the HBsAg rose rapidly with age, reaching peak values (5·3%) at 20–29 years, while anti-HBs reached its highest value (42·1%) in older age groups (50–59 years). The frequency of HBsAg was significantly higher in males (3·4%) than in females (1·5%). The percentage of infected persons who become chronic HBsAg carriers (12·3%) was found higher than in other developed populations. It was also found higher in children (25%) than in adults (5·5%) and in males (14·6%) than in females (8·1%). These data indicate that HBV infection in early life is a major risk factor in the development of HBsAg carriers and support the hypothesis that males are more likely to become HBsAg carriers than females.