The "rave" phenomenon—loud music, flashing lights, and frenzied all-night dancing—has been a major element in the resurgence of psychedelic drug use in Western society.1 Purportedly central to raves is the use of "club drugs," including 3,4-methylenedioxy-methamphetamine, also called MDMA or ecstasy.1 The use of ecstasy seems to be increasing worldwide, with rave attendees being a high-risk population. Because these drugs have potentially serious physical and psychological consequences, such as anxiety, memory loss, paranoia, depression, cognitive impairment, cardiac complications, and kidney failure,2- 3 the rave phenomenon has sparked the attention of health officials and policy makers. However, to our knowledge, no studies have collected self-report or objective drug use information from rave attendees in the United States.