The accuracy of reports compared to clinic records varied by drug, with the percentage of known users reporting their use highest for marijuana, followed by cocaine and hallucinogens, and lowest for heroin. Almost half of this sample of former treatment clients denied ever receiving drug treatment. Threats to the validity of self-reported drug use include response distortion to avoid social stigma and fear of legal consequences. Results of this study indicate that underreporting of drug use increased as the social stigma associated with the drug increased. For purposes of this study, the findings of earlier studies suggest that former drug treatment clients generally appear willing to report past drug use, but that questions about past-year drug use may be subject to measurement error related to the cognitive complexity and demands on recall of these items. Tables, references