Congenital toxoplasmosis may develop after maternal primary infection during pregnancy. Since toxoplasmosis in adults is usually asymptomatic, the only way to find the pregnancies at risk of acute toxoplasmosis (i.e. seronegative mothers) is to perform serological screening of all pregnant women. We studied toxoplasma-specific antibodies in 16,733 pregnant women in the first, second, and third trimesters over an 18-month period in the Helsinki area, with a population of 820,700. The participation rate in our study was 90.2% of all pregnancies in the area. The overall prevalence of toxoplasma seropositivity was 20.3%. The incidence of primary infection was 2.4/1000 pregnancies at risk. Our data on incidence suggest an annual occurrence of 131 cases of primary toxoplasma infections during pregnancy in Finland. Primary toxoplasma infection appears to be a significant risk to the fetus even in countries where the prevalence of toxoplasma seropositivity is low. The feasibility of mass screening for toxoplasma infections during pregnancy should be considered in order to reduce the morbidity and mortality due to congenital toxoplasmosis.