A prospective survey of antenatal patients was made at a hospital in Brussels over the period 1979-1982 to assess the incidence of congenital toxoplasmosis. Of 2986 patients assessed, 1403 (47%) had no toxoplasma antibodies and were at risk. The susceptible population was assessed every 6 weeks and 20 of these women (1.4%) seroconverted during pregnancy. Ten of the seroconverters had a therapeutic abortion; of the remaining 10 two gave birth to congenitally infected infants. Of the 1583 (53%) patients with a positive serology for toxoplasmosis initially, 17 (1.1%) had high antibody titres indicating that the infection could have taken place early in pregnancy. Nine infants born to these women were followed at our hospital and two of them were affected.