The design of a two year prospective study of 45,000 mothers and their newborn children aiming at determining the incidence of congenital toxoplasmosis in Denmark is presented. Toxoplasma IgG antibodies in a bloodspot from the newborn, routinely collected on filterpaper (the Guthrie card), and IgG antibodies in a stored serum sample from the mother collected in the first trimester are compared. Seroconversion identify mothers infected with T. gondii during pregnancy and identify newborn children at risk of congenital toxoplasmosis. All children at risk will be treated and followed until congenital toxoplasmosis is verified or excluded. Children with verified congenital toxoplasmosis will be followed clinically and the effectiveness of the treatment will be evaluated. The study design allows determination of the materno-fetal transmission rate in untreated pregnancies, and will furthermore allow a good estimate of the incidence of congenital toxoplasmosis in Denmark. In addition we hope that the diagnosis and treatment of cases of subclinical toxoplasmosis will prevent chorioretinitis and minor cerebral disorders later in life. Of 6355 pregnancies tested during the first 3 month of the study, 14 cases of maternal seroconversions have been diagnosed. Only one child had detectable toxoplasma specific IgM antibodies at birth.