In recent years, club drugs such as MDMA, Ketamine, GHB, and Rohypnol have emerged as major drugs of abuse. The national and local Chicago news media have publicized law enforcement actions and adverse health outcomes, including fatalities, related to the abuse of these substances. Media accounts and a limited body of research have identified use of these substances as prevalent in the gay male community. This prevalence coincides with recent increases in HIV seropositive incidence. There is a clear need for a more comprehensive understanding of the prevalence of club drug use in the general population, and particularly in the subgroup of sexually active gay men. Noting these research gaps and their considerable adverse public health implications, this supplemental study was designed to apply an expanded protocol developed from an earlier study conducted (Feasibility and Use of Biological Measurement in Drug Surveys; R01DA12425, SRL Study #860) to a sample of gay men in Chicago. This study evaluated whether findings regarding the feasibility and use of drug testing in drug surveys derived from general population samples are generalizable to a probability sample of 216 gay men in the city of Chicago. For this project, a supplemental module was added to the main study survey that asked detailed questions about involvement in the gay community, risky sexual activity and HIV seropositivity. The scope of biological measurement was also expanded to incorporate testing for Rohypnol and Ketamine in hair (MDMA was already being tested as part of the general sample hair screen).