A method was developed for measuring cocaine and its metabolites, benzoylecgonine, ecgonine methyl ester, norcocaine, ecgonine ethyl ester, cocaethylene, and m-hydroxybenzoylecgonine, in breast milk by gas chromatography/mass spectrometry. Limits of detection for this method ranged from 2.5 to 10 ng/mL, and limits of quantitation ranged from 5 to 50 ng/mL. For each of the compounds measured by this method, linear response was demonstrated to 750 ng/mL. Breast milk was collected from 11 mothers who admitted to drug use during pregnancy and ten drug-free volunteers serving as control subjects. Cocaine was detected in six of the specimens obtained from drug-exposed subjects, and in none of the drug-free control subjects. In breast milk specimens where cocaine and one or more of its metabolites were detected, the concentration of parent compound was greater than any of the metabolites. The highest cocaine concentration found was over 12 microg/mL. Breast-fed infants of cocaine abusing mothers may be exposed to significant amounts of drug orally.