The use of hair analysis for assessing therapeutic compliance is endowed with certain limitations, but there are potential advantages that supersede the limitations of existing techniques. There is now unquestionable evidence that drugs become incorporated into hair. The majority of drug incorporated into hair is of systemic origin during the active growth phase although the possibility of passive incorporation from sweat cannot be totally ignored. The major advantage of the hair analysis technique over those currently available is its potential to provide retrospective information about a patient’s drug taking behavior. In addition, its noninvasiveness, the stability of the drug within the matrix, and the unnecessary need to centrifuge or freeze the sample post-collection makes it a particularly attractive option for multicentered outpatient clinical trials. Although there are established variables in the nature of hair within the individual, these parameters are significantly less variable. The fact that hair growth is dependent upon the race, sex, and age of an individual as well as its anatomical location is an additional complication if comparisons are being made between individuals. But because compliance is being assessed for an individual, these variables are negated. It is important to standardize the location of sample collection. The primary reason why samples are collected from the posterior vertex is that in this region at any one time, about 85 percent of the hair follicles are in the active growth phase. These values can be affected by nutritional status, disease states, and other drugs, but within the individual such influences would be relatively consistent over time. The greatest source of variation within the individual is the effect of chemically or cosmetically treating the hair. Because these procedures affect the physical structure of the hair and the stability of incorporated drugs, interpretation has to be performed with care. Hair analysis can potentially detect a variation in drug taking behavior, which is by far the most common form of noncompliance. 3 tables, 10 figures, 60 references