Hair specimens were collected from 322 subjects and analyzed as part of an experimental study administering household surveys during 1997 to a high-risk community sample of adults from Chicago, Illinois. Toxicologic results were compared with survey responses about recent and lifetime drug use. About 35% of the sample tested positive for cocaine, and 4% tested positive for heroin. Sample prevalence estimates of cocaine use based on toxicologic results were nearly five times the survey-based estimates of past month use and nearly four times the survey-based estimates of past year use. With the hair test results as the standard, cocaine and heroin use were considerably underreported in the survey. Underreporting was more of a problem for cocaine than for heroin. Among those who tested positive, survey disclosure of cocaine use was associated with higher levels of cocaine detected in hair. In general, when recent drug use was reported, it was usually detected in hair. When a drug was detected in hair, use was usually not reported in the survey. When heroin was detected in hair, cocaine was almost always detected as well. Am J Epidemiol 1999; 149:955–62.