The addition of two new markers in maternal serum, estriol and HCG, to those already known, namely the level of maternal serum alfa-fetoprotein and maternal age, considerably improves the expected results of a screening strategy for Down syndrome. The detection rate is slightly increased from 53.0% to 57.6%, but, more importantly, the false-positive rate decreases from 9.4% to 7.3%. It is our belief that, at least in women aged less than 35 years, a screening strategy based on a combination of maternal age and biochemical markers should be incorporated into antenatal care. For older women, the results of such a maternal serum test may refine counseling for genetic amniocentesis, as a much more explicit risk calculation can be performed than that based on age alone.