There have been recent alarming increases in Ecstasy use and growing evidence of possible long-term neurological and cognitive effects. This article considers Ecstasy use within its common polydrug context (specifically with methamphetamine [MA]), examining differences between Ecstasy-using and nonusing subgroups of clients treated for MA use, and exploring the relationship of Ecstasy use to selected treatment outcomes. Multivariate logistic regression showed Ecstasy+MA users differing from MA users who have not used Ecstasy primarily in terms of sociodemographics (higher income, fewer children), substance abuse behaviors and motivators (lifetime history of more types of drugs, more likely to report use of Ecstasy to enhance sex, more drug-related problems), lifestyle (more likely to have had same-sex sex partners), and treatment characteristics (younger at admission, less likely to complete treatment). A lower rate of treatment completion was predicted by Ecstasy use, even when controlling for other Ecstasy-related characteristics; however, time to relapse was not significantly related to Ecstasy use. These results distinguishing subgroups of stimulant users may help guide additional research into specific behavioral causes and correlates of Ecstasy use, which could in turn guide specialization of promising MA-related treatment approaches for Ecstasy users.