After ingesting 3,4-methylene-dioxy-methamphetamine (MDMA) and the monoamine oxidase (MAO) inhibitor phenelzine, a 50 year old male developed marked hypertension, diaphoresis, altered mental status, and hypertonicity lasting 5-6 hours. This clinical course is typical of interaction between MAO inhibitors and some sympathomimetics including amphetamines. Such interaction has not previously been described involving MDMA. Sympathomimetic-MAO inhibitor interactions can cause excessive release of endogenous bioactive amines (e.g. norepinephrine, serotonin). Hypertensive crisis, intracranial hemorrhage, hypertonicity, and severe hyperthermia have occurred due to sympathomimetic-MAO inhibitor interactions. MDMA shares structural and pharmacologic features with other agents capable of causing this interaction, and this case suggests that MDMA can cause significant toxicity in patients taking MAO inhibitors.