In response to a Congressional request, the General Accounting Office (GAO) investigated drug use measurement by reporting the drug use patterns of targeted groups in three nationally prominent drug studies, assessing the methodological strengths and limitations of the studies, and developing recommendations for the improvement of drug prevalence estimates. The GAO examined the National Household Survey on Drug Abuse (NHSDA), the High School Senior Survey (HSSS), and the Drug Use Forecasting (DUF) study of booked arrestees. The NHSDA was found to be limited by the exclusion of groups at high risk for drug use, problematic measurement of heroin and cocaine use, and reliance on subject self-reports. HSSS excluded dropouts and absentees, yielded questionable estimates of drug use in non-white populations, and relied on self-reports. DUF employed both self-reports and the objective technique of urinalysis for assessing drug use, but its findings cannot be generalized. This document presents the report GAO sent to Congress concerning the evaluation of the three drug use studies and the recommendations for improving the studies. Following an executive summary, chapter 1 of the report provides an introduction to the GAO work. Chapter 2 describes the NHSDA, HSSS, and DUF studies and illustrates the drug use rates for marijuana, cocaine, and heroin and other opiates for different target groups. Chapter 3 describes the strengths and limitations of each of the three studies. Chapter 4 provides guidelines and recommendations for enhancing drug prevalence estimation techniques. (NB)