Cocaine use during pregnancy is difficult to ascertain, and maternal reports are likely to be inaccurate. Presently, the dose-response characteristics between maternal cocaine use and fetal exposure and adverse effects are unknown. Clinically, some babies are harmed, whereas others are not adversely affected. Taking advantage of the fact that cocaine and its metabolite benzoylecgonine (BE) accumulate and can be detected months after exposure in maternal and neonatal hair, an analytical test for cocaine and BE was developed by the authors. The aim of this study was to describe the characteristics of maternal and neonatal hair cocaine as biomarkers of fetal exposure. Of nearly 10,000 cases, all mother-child pairs in whom at least one had cocaine and/or BE detected in hair were identified. The relationship between maternal and neonatal levels was studied. When available, these data were also compared with meconium levels of cocaine. Median cocaine concentration was 10-fold higher in hair of the mothers compared with the neonates (3.56 ng/mg vs 0.31 ng/mg of hair). Infants' cocaine in hair was positively correlated with maternal cocaine and BE in hair (r2 = 0.41 and r2 = 0.22, respectively, P < 0.001 for both correlations). Infants' BE was also correlated with maternal cocaine and BE concentrations in hair (r2 = 0.50 and r2 = 0.27, P < 0.001 for both correlations). Thirty-nine (40%) babies had negative cocaine and BE results despite their mothers being positive. Mothers whose infants were cocaine-positive had a median hair cocaine concentration of 7.34 ng/mg, significantly higher than those whose infants were negative (1.25 ng/mg). Maternal cocaine levels below 0.24 ng/mg may serve as a relative threshold for detectable fetal exposure during the third trimester of pregnancy. Fetal hair grows in the last trimester. Hence, a positive neonatal hair indicates maternal use after pregnancy became known, a strong indicator of maternal addiction. Transplacental exposure to cocaine of babies of addicted mothers is highly variable. The dose-response relationship of both cocaine and BE between maternal and neonatal hair suggests that the placenta protects some fetuses but not others. More research is needed to understand the mechanisms leading to placental defense against cocaine.