Background. The favorable attitude among the public towards prenatal diagnostics in Finland allowed us to start a trial on population basis when screening for Down's syndrome by maternal serum markers and age was introduced. Methods. Screening by maternal serum markers for Down's syndrome was offered to all 17 200 pregnant women in the Helsinki area during the study period of 2.5 years. Screening due to advanced maternal age, 37 years or more, was continued as previously, and 1133 pregnant mothers used this option. Alpha-fetoprotein, human chorionic gonadotrophin, and during the first year also unconjugated estriol were used as markers. Results. The uptake of serum screening was 84%. The proportion of false positive results i.e. risk for Down's syndrome, 1:350 or more at term, was initially 5.7%. After ultrasound scan 4.1% of the mothers remained ‘screen positive’. The amniocentesis or chorionic villus sampling uptake was 98.4%. Ten out of eighteen cases of Down' syndrome were detected by maternal serum screening, sensitivity 56%, 95% CI 31-79%. Other chromosomal abnormalities were found in three cases, and there were four cases of mosaicisms confined to the placenta. These were trisomies 16, 7 and 2, and tetraploidy. Elevated serum alpha-fetoprotein was found initially in 0.7% of the cases. One case of congenital nephrosis of the Finnish type and ten other, mainly structural, abnormalities were detected by high AFP. Conclusions. The screening was well received by the mothers. The detection rate of 56% is in the same range as in previous studies. Ultrasound scan before the test would effectively lower the false positive rate caused by incorrect timing.