Testing the newborn is important for the identification of intrauterine drug exposure. For recent exposure, the test is usually done on a urine specimen; however, it is possible to get a false negative result. Testing the meconium, or the baby’s first stools, has limitations. Hair analysis is significantly more sensitive in detecting in utero exposure to cocaine. A major advantage of hair testing compared to urine or meconium testing is that the timing of hair collection is not critical. Hair collected even a few weeks after birth offers the likelihood of detecting gestational drug exposure, assuming environmental contamination can be ruled out. If the mother abstains from drugs for a few days before delivery, the baby’s urine will test negative, but the hair analysis will be positive. A prevalence study of cocaine use during pregnancy was conducted in Toronto between June 1990 and December 1991. Hair and urine samples were collected from 600 infants. Data suggest that exposure below a certain level cannot be detected in fetal hair; without testing the hair, 76 percent of the positive cases would have been missed; and women whose history revealed a voluntary report of cocaine use were likely to be heavier users. Hair testing is also an easy and noninvasive way to identify external exposure to drugs, such as environmental tobacco smoke (ETS) exposure. Pharmacokinetic predisposition to tobacco smoke in certain children may lead to a higher risk for asthma because of a larger exposure to a given amount of cigarette smoke. Care must be taken to use the most appropriate specimen and the most sensitive methods to corroborate or refute suspicion of drug exposure. Hair analysis has become such a powerful tool in diagnosing intrauterine drug exposure and ETS exposure in children that it is quickly gaining in popularity. 79 references