Recent reports document high rates of asymptomatic hepatitis B virus infection in pregnant Hispanic women of Caribbean and Latin American origin, frequently in the absence of identifiable risk factors. We hypothesized that the prevalence of asymptomatic hepatitis B virus infection in Mexican-American women was much lower and that most belonged to established risk groups. Three thousand seven hundred eight-nine pregnant women, 77% of whom had Hispanic surnames, were screened for hepatitis B surface antigen upon admission in labor to Medical Center Hospital in San Antonio. Twelve women, six of whom had Hispanic surnames, were found to have asymptomatic hepatitis B infections. The prevalence of asymptomatic infections was 3.2 per 1000 (95% confidence interval 1.6-5.5) in the total population, 2.0 per 1000 (95% confidence interval 0.7-4.5) in those with Hispanic surnames, and 7.0 per 1000 (95% confidence interval 2.5-15.0) in those with non-Hispanic surnames. Risk factors, as defined by the Centers for Disease Control, were found in five (42%) of the positive patients overall and in only one (17%) of the positive patients with an Hispanic surname. We conclude that, although asymptomatic hepatitis B infection is uncommon in these pregnant Mexican-American women, the absence of identifiable risk factors in the majority of those infected suggests that routine screening in this population is justified.