BACKGROUND: Primary infection with Toxoplasma gondii during pregnancy can cause fetal infection with a risk of complications for the fetus. The proportion of women at risk of acquiring the infection during pregnancy in Sweden is not known. METHODS: The seroprevalence of Toxoplasma gondii among pregnant women in Sweden was calculated when 40,978 blood samples collected on filter paper eluates from newborns were tested for Toxoplasma specific immunoglobulin G. The newborns in two geographically different areas (Stockholm County and Skåne County) were investigated during a 16-month period between April 1997 and July 1998. The anti-toxoplasma IgG antibodies detected in the eluates were considered to be of maternal origin and to reflect the immune status of the mother. RESULTS: The seroprevalence was 14.0% in Stockholm County and 25.7% in Skåne County. The seroprevalence among women born in the Nordic countries was 11.1% in Stockholm and 24.9% in Skåne. The corresponding figures for women born outside the Nordic countries were 24.3% and 29.4%. On comparing the seroprevalence found in this study with older data, the overall seroprevalence in pregnant women born in the Nordic countries and living in Stockholm was found to have decreased between 1969 and 1998. The seroprevalence in different birth cohorts, longitudinally followed in the previous and the present studies, remained at the same level over 20 years despite the increasing age of the women. CONCLUSION: These results suggest that the majority of seropositive pregnant women in Sweden today have seroconverted before entering the childbearing period and that the percentage of women in Sweden acquiring toxoplasmosis during the childbearing period is low.