Turner syndrome, occurring in 1:2500 female births, is caused by the complete or partial absence of one X chromosome. Amniotic fluid supernatant proteins from five second trimester pregnancies with Turner syndrome fetuses and five normal ones were analyzed by 2DE, MALDI-TOF-MS, and Western blot. Serotransferin, lumican, plasma retinol-binding protein, and apolipoprotein A-I were increased in Turner syndrome, while kininogen, prothrombin, and apolipoprotein A-IV were decreased. Since differentially expressed proteins are likely to cross the placenta barrier and be detected in maternal plasma, proteomic analysis may enhance research for noninvasive prenatal diagnosis of Turner syndrome.