We present a case of acute, symptomatic hyponatremia in a young woman that developed after use of 3,4-methylenedioxymethylamphetamine (MDMA), more commonly known as "ecstasy." The patient was treated with 5% saline and had complete recovery. The pathogenesis of MDMA-associated hyponatremia involves excessive water intake and inappropriately elevated antidiuretic hormone (ADH) levels. It seems that young, premenopausal women are at particularly high risk for the development of severe, symptomatic hyponatremia after use of this drug. Review of the literature revealed 4 fatal outcomes from MDMA-associated hyponatremia. All were women and all died from cerebellar tonsillar herniation. We suggest that acute hyponatremia that develops after MDMA use may be a life-threatening condition. Recent recommendation that MDMA users should drink large volumes of water may not be appropriate.