To determine the extent of fetal exposure to cigarette smoking, we measured the concentrations of nicotine and cotinine in maternal and neonatal hair. In this study of 19 mother-infant pairs, strands of hair were collected from mothers and neonates after birth. These samples were digested by protease, and the nicotine and cotinine concentrations were analyzed by high performance liquid chromatography. In actively smoking mothers, nicotine and cotinine were detected in maternal and neonatal hair. The concentrations of nicotine were 29-398 ng/10 mg of hair in mothers and 11-63 ng/10 mg of hair in neonates. The concentrations of cotinine were 1.7-22.0 ng/10 mg of hair in mothers and 1.6-11.2 ng/10 mg of hair in neonates. In non-smoking mothers with smoking families, nicotine and cotinine were detected in the hair. Hair concentrations of nicotine and cotinine in neonates can be used as a biological marker of the extent of intrauterine exposure to cigarette smoking.