Drug self-reports are often unreliable and standard blood and urine tests detect only recent cocaine use. Since cocaine is deposited in hair, we have applied a radioimmunoassay to hair extract to detect past cocaine use. Hair from 16 adult users was positive for benzoylecgonine, in the presence of negative findings from urine screening tests. Benzoylecgonine in admitted heavy users averaged 8775 ng/g of hair (range, 640 to 29 089 ng/g of hair), whereas in occasional users it averaged 624 ng/g of hair (range, 32 to 1210 ng/g of hair). Benzoylecgonine was not detected in hair of 21 adults who reported no use of cocaine ever and whose urine samples were negative for the metabolite. Neonatal hair from seven infants whose mothers were known cocaine users averaged 5430 ng of benzoylecgonine per gram of hair (range, 200 to 27500 ng/g of hair). Hair from two infants 2.5 and 3.5 months of age averaged 6050 ng of benzoylecgonine per gram of hair. However, values were negative for infants 1 year and older, corresponding to loss of fetal hair in the few months after birth. Because studies reporting reproductive risks of cocaine compare exposed and nonexposed groups, validation of drug-free status of control subjects is extremely important. Hair analysis may remedy the disadvantages of currently used methods and may identify intrauterine exposure to cocaine in babies when a maternal drug history is not available or of doubtful truthfulness.