A national study was performed to assess the seroprevalence of anti-Toxoplasma gondii IgG in pregnant women at the time of delivery. The study was organized between 1990 and 1991 in 23 out of 26 Swiss cantons. 9059 women, corresponding to 11.8% of the annual total of births in those cantons, were included. The global seroprevalence of specific IgG was 46.1% (95% confidence interval: 45.0-47.1%). There was no significant difference in seroprevalence between different cantons after adjustment according to age. At the national level, the seroprevalence was 46.0% for Swiss women and 45.8% for women of other nationality (information on nationality was available only for 8382 persons). The use of a model of linear regression according to age showed that the risk of seroconversion among seronegative women during their 9 months of pregnancy was 1.21%. In addition, a certain number of data were calculated from that value to "simulate" a theoretical situation with absence of specific serological screening during pregnancy at a national level and consequently, absence of appropriate treatment. It was estimated that 548 cases of seroconversion would occur annually during pregnancy. This would lead to 183 congenital transmissions of toxoplasmosis, among which, 75% would be asymptomatic at birth. The number of expected pathologies would be: 40 cases of chorioretinitis with impared vision, 18 cases of cerebral lesions, and 2.7 cases of perinatal death. We observed positive results for specific anti-T. gondii-IgM in 1.7% of persons tested. This result can be the source of medical interpretation difficulties if the serum sample is the first done during pregnancy.