The prevalence and natural course of chronic hepatitis C virus (HCV) infection was evaluated in 15,250 consecutive pregnant women. The rate of HCV vertical and perinatal transmission was also assessed. The presence of anti-HCV was tested by means of EIA III and confirmed by recombinant immunoblot assay III. Alanine transaminase (ALT), anti-human immunodeficiency virus (HIV), and HCV-RNA were tested during the first month and third trimester of pregnancy, and 6 months after delivery; the same tests were made in all of the newborns of anti-HCV-positive mothers at birth (on cord blood samples) and then at 4-month intervals. Anti-HCV positivity was found in 370 cases (2.4%), 72% of whom were also HCV-RNA-positive. The proportion of women with hypertransaminases decreased from 56.4% at the first examination during the first month of pregnancy to 7.4% in the last trimester, and then increased again after delivery (54. 5%), without any concomitant changes in the proportion of those with viremia. The proportion of anti-HCV- and HCV-RNA-positive newborns was 5.1% after 1 year (8 of 155), all of whom had the same genotype as their mother. The rate of HCV transmission was not affected by the type of delivery or feeding, or the HIV status of the mother. The results of this large-scale study confirm previous data in smaller series concerning the prevalence of HCV infection in pregnant women, and strongly support the hypothesis of a favorable (possibly immunomediated) effect of pregnancy on liver cell necrosis in anti-HCV-positive women.