MDMA (or 3, 4 methylenedioxymethamphetamine) was first manufactured in the 1920s and found to have structural similarities to both mescaline and amphetamines. Used briefly by some therapists in the 1970s and early 1980s as an adjunct to psychotherapy, it is now primarily abused by teenagers and young adults as an illicit recreational drug known as "ecstasy." As its popularity has increased, so have the number of fatalities and adverse events related to its use. We report six patients suffering fatal or life-threatening hyperthermia after MDMA use. These cases illustrate that hyperthermia associated with MDMA use cannot be solely attributed to rave parties (high ambient temperatures, excessive dancing, dehydration, and overcrowded conditions), drug contaminants, or co-ingestants. A better understanding of the etiology of hyperthermia after MDMA use is needed so that appropriate harm-reduction measures can be developed and instituted.