The median maternal serum free beta human chorionic gonadotropin (hCG) multiple of the median (MOM) of 480 Down syndrome cases in the second trimester was 2.64, significantly greater than the reported median MOM of intact hCG (p < 0.0001). In 234 of these cases from retrospective and prospective studies, the effectiveness of maternal serum free beta hCG was evaluated in combination with alpha-fetoprotein (AFP) and maternal age in second-trimester Down syndrome screening. Down syndrome detection in the gestational age range of 14-16 weeks was 82 per cent. In all gestational weeks (14-22), a 77.7 per cent Down syndrome detection rate was achieved. In prospective screening of 44,272 patients under the age of 35 years, 69 per cent of Down syndrome cases were detected (73 per cent in gestational weeks 14-16). The false-positive rate for the prospective study was 3.8 per cent. The use of free beta hCG combined with maternal serum AFP and maternal age-related risk for Down syndrome in a screening population (i.e., women under 35 years) yields an improved detection efficiency over other protocols.