The aim of this prospective study was to compare triple test screening (alpha-fetoprotein, beta-chorionic gonadotrophin and unconjugated oestriol) with amniocentesis in the detection of fetuses with Down's syndrome in women of 35 years of age or more. Between 1992 and 1996, maternal serum markers were evaluated in 1406 women who had amniocentesis for prenatal diagnosis of chromosomal abnormalities related to a maternal age of 35 or more years. Sixteen fetuses with Down's syndrome were identified in the whole group by amniocentesis and karyotyping. The group with negative triple test screening consisted of 919 pregnancies and included two fetuses with trisomy-21 (false negatives). With triple test screening in the age group over 35, there was a detection rate of 87.5% for cut-off points ranging from 1:200 up to 1:350, with corresponding false positive rates ranging between 23% and 34%. In our population, if we had practiced the policy of offering amniocentesis only to women screening positive for the ages of 35 and 36 and to all pregnant women of 37 or more, we would have carried out 30% less amniocenteses. In this group of 1406 women, 33 abnormal karyotypes were detected with amniocentesis (16 Down's syndrome included) and equal number of elective abortions were carried out. Nevertheless, 19 healthy fetuses and neonates were lost after amniocentesis. Considering the high detection rates that can be achieved with triple test screening, the existing procedure related risk of amniocentesis (0.5-1.0%), and the facts that conception in women over 35 years of age is usually more difficult and the background loss usually higher than in younger women, we believe that in the future women over 35 should be offered a choice between non-invasive and invasive procedures after being thoroughly informed.