Labeling an individual a drug abuser has serious sociologic and legal implications that only laboratory testing can effectively allay. A proper specimen (both qualitatively and quantitatively) must be obtained for analysis. Positive identification of specimen with subject is of paramount importance. The problems of specimen substitution--other people's urine, tap water, apple juice--directly impinge here, as does the possibility of drug degradation by heat, light, and microbial attack and of drug adsorption by the container and urinary sediment. Confirmation of postives indicated by screening tests (thin layer chromatography and immunoassays) by gas chromatography and/or ultraviolet spectrophotometry is, in most situations, mandatory. An effective quality control program is an absolute requirement. Even under ideal circumstances, laboratory results can sometimes wrongly indicate the abuse of drugs; and conversely, drug abuse can take place without detection by the laboratory. As in any clinical situation, laboratory tests are only a part (albeit an important one) of the entire evaluation of the individual involved.