Some drug researchers, however, are troubled by the technology of hair analysis for drugs of abuse because questions remain about variables affecting drug sequestration in hair, the comparability of results based on different drug extraction techniques, and the reliability and quantifiability of analysis procedures. These researchers note that the uptake of cocaine and its metabolites into hair is not a strict function of blood concentration. When pregnant or postpartum women's hair contains cocaine, it is helpful to know the meaning of hair concentration for infant exposure and treatment needs. Although infant hair analysis for cocaine should have high specificity, infant hair analysis provides no direct information about first trimester exposure. Therefore, the best estimate of early gestational exposure of infants depends on the valid and reliable analysis of maternal hair. The greatest value of maternal hair testing may be that it allows the researcher or the program evaluator to collect an integrated pool of drug use outcome measures that represent an extended post-treatment period. Hair testing may answer questions about long-term exposure when serial urine sampling is not a practical alternative because of its cost and inconvenience. Hair analysis techniques are described and their correlation with other measures is examined. 30 references, 1 table, and 2 figures