We investigated the levels of S100 beta protein (S100B) in the serum of fetuses with neural tube defects (NTD), and their mother. Samples from 20 fetuses with NTD and 30 controls at the same gestational age, and their mothers, were studied. S100B protein levels were determined using Lia-mat Sangtec. kit. S100B concentrations were significantly higher in NTD fetuses (median 2.71 microg/L) than in control subjects (median 0.98 microg/L). Increased S100B levels were also found in mothers carrying fetuses with NTD compared to control and uncomplicated pregnancies. This study indicates that NTD is associated with increased serum concentration of S100B in fetuses and mothers. Moreover, it gives information on S100B levels in the fetal circulation in early-mid gestation.