Fourteen cases of Turner syndrome (45,X), two cases of mosaic Turner syndrome (45,X/47,XXX and '45,N 46,XX), and one case of Turner syndrome involving an isochromosome X [46,X,i(X)(qlO)] were ascertained by prenatal maternal serum alpha-fetoprotein (MSAFP) and free beta human chorionic gonadotropin (hCG) screening or by ultrasound. Patient-specific risks for Down syndrome were calculated and used as the criteria to determine offering further testing. Eleven of the 17 cases had hydrops and presented with an increased Down syndrome risk based on MSAFP and free beta hCG screening. The median MOM level was 0.98 and 4.04 for MSAFP and free beta hCG, respectively. Three cases had hydrops but screened negative. The two cases of mosaic Turner syndrome were non-hydropic and screened positive. The 46,X,i(X)(q10) case was non-hydropic but had elevated MSAFP and free beta hCG levels. These data suggest that Turner syndrome pregnancies do not appear to screen positive due to hydrops alone, but screening may also be influenced by the inherent genetic imbalance in the fetus and placenta. Because the MSAFP levels in our series were within the normative range in all except one case with an elevated MSAFP, free beta hCG alone was the most effective screening marker for Turner syndrome pregnancies.