For prenatal screening of chromosomal aneuploidies (primarily the most frequent Down syndrome) maternal serum AFP screening in the second trimester of pregnancy has been supplemented by the determination of hCG in Hajdú-Bihar county. In pregnancies at risk on the basis of biochemical tests, a thorough, aimed ultrasound examination for the detection of minor and major anomalies characteristic for chromosomal abnormalities was carried out. If both biochemical and ultrasound examinations were suggestive of high risk prenatal karyotyping was offered. During a two-years prospective study 14328 pregnancies were screened. Authors could detect 38% of Down-syndrome cases prenatally, 5 cases in pregnant women at age under 35 years and at the same time the number of amniocenteses increased only by 70. It was concluded that 66% of all Down syndrome cases could have been prenatally diagnosed if prenatal chromosome test were performed in all cases at high risk on the basis of screening tests and maternal age.