The prevalences of hepatitis B surface antigen (HBs Ag) subtypes in Thais, Cambodians, and Vietnamese were compared with the prevalences in Americans residing in Southeast Asia. HBs Ag was found with approximately equal frequency in Thai (43%) and American (39%) patients with hepatitis. However, higher prevalences of HRs Ag were found in asymptomatic Thais (9.5%), Cambodians (11.9%), and Vietnamese (14.3%) than in asymptomatic Americans (0.7%). Among asymptomatic Thais, the ratio of HR s Ag/adr to HBs Ag/adw was approximately 10:1, with one exception: adw was not detected in a rural population of northern Thailand. The y determinant was not found in Thais. In contrast, both d and y determinants were found in Americans. These observations conform to a geographic pattern, with ad as the predominant combination in the Far East. In Southeast Asia determinants wand r are more useful epidemiologic markers than y and d.