We present a genetic and epidemiological study of trisomy 21 (T21) in the Canton of Vaud, the area covered by our local registry of congenital anomalies which has participated in EUROCAT Switzerland since 1988. During the period 1980-1996, we found 240 new T21 cases, all cytogenetically proven, out of 115,064 consecutive live births. Our purpose was to study trends and impact of biochemical screening and prenatal diagnosis of T21. We considered two different periods: 1980-1989 (before biochemical screening) and 1990-1996 (with screening) during which the mean maternal ages were respectively 28.4 years (10.6% > or = 35) and 29.2 years (12.9% > or = 35). The total prevalence of T21 was 2.08 per 1000; 5.4% of the cases were stillbirths, 49.6% were induced abortions and 45% livebirths. Prenatal cytogenetic diagnosis of trisomy 21 was performed in 52.1% of cases. Among women aged 35 or over the prenatal detection rates are superposable in the two periods. However, for younger women this rate has been much higher since the introduction of biochemical screening, i.e. 9.8% before and 51.8% after the introduction of triple test. In conclusion, the increase in prenatal diagnosis tests performed because of abnormal maternal serum marker levels has increased the global prenatal detection rate from 36.6% to 63.3% in our population, and the prevalence of Down syndrome has thus slightly decreased among livebirths.