Recent studies of treated and untreated populations using improved urinalysis techniques as well as hair analysis techniques indicate that the validity of respondents' self-reports of recent drug use may be considerably less than previously reported and may differ according to a number of factors. At intake, almost all clients of the Washington, DC, study who tested positive had reported their use of heroin but fewer clients had reported their cocaine use. At posttreatment followup, clients underreported both heroin and cocaine use. The main results of this study include the following: (1) Validity of self-reports of drug use may differ by drug; (2) Multiple drug use may go undiagnosed by self-report measures; (3) Hair analysis detected more cocaine use than did urinalysis; (4) While some clients underreport drug use, their disclosures of extensive drug use may still have substantial validity; (5) Hair analysis may offer some diagnostic utility; (6) Validity of self-reports of recent drug use may be less at followup than at intake; and (7) Underreporting may be less of a problem among the most serious substance abusers. Tables, figures, references