Cord blood screening for congenital toxoplasmosis was performed prospectively on 30,000 samples collected between 1986 and 1994 in the region of Basel, Switzerland, covering up to 95% of all births. Congenital infection was suspected in cases of serum with specific antitoxoplasma IgM or IgA or with a level of specific IgG above 300 IU/ml as measured by EIA from Sanofi-Pasteur. The percentage of cases to be screened declined from 2.5 to 1.2% and the cases of confirmed congenital toxoplasmosis from 0.073 to 0.033% during the observation period. This observation may be due to several reasons, such as improved primary prevention or more frequent diagnosis and treatment during pregnancy. In 178 cases of cord blood serology suspicious for acute toxoplasmosis, an enquiry with the gynecologist in charge of the pregnant woman was carried out. 126 mothers (71%) presented with a confirmed immunity against Toxoplasma gondii early in pregnancy and screening of the child was not needed. From 37 pregnancies (21%) no information was available. 15 out of 24 confirmed cases of toxoplasmosis of the mother during pregnancy were detected by cord blood analysis. In 17 of 24 cases of maternal toxoplasmosis treatment with spiramycin, pyrimethamine/sulfadoxine or both was performed. 10 of 17 newborns from mothers treated during pregnancy had elevated toxoplasma IgG titres at birth, but only one child was infected. 7 newborns had an inconspicuous toxoplasma serology at birth and were not infected. 7 cases of toxoplasmosis during pregnancy were detected by the check back and remained untreated; 4 out of 7 newborns had congenital toxoplasmosis.(ABSTRACT TRUNCATED AT 250 WORDS)