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Study objective: To describe the clinical characteristics of patients with Ecstasy (MDMA)-associated hyponatremia (serum sodium less than 130 mmol/L) reported to the California Poison Control System (CPCS) over a five-year period and to determine if a gender difference exists among patients with Ecstasy-associated hyponatremia. Methods:We performed a retrospective review of cases involving Ecstasy intoxication reported to the CPCS and recorded in its computerized database from January 1, 2000 through October 9, 2005 We excluded all "information only" calls, cases managed at home, cases with no gender specified and cases in which the outcome was coded as "no effect", "unrelated -probably not responsible", "confirmed non-exposure", "judged as non-toxic-expect no effect", and "minimal clinical effects possible". Confirmation of exposure to MDMA was based on the history of use and, when available, urine toxicology screen testing positive for MDMA or amphetamine derivatives. Hyponatremia was defined as a measured serum sodium less than 130 mmol/L.Results: A total of 1,436 cases involving Ecstasy were reported to the CPCS over the 5-year study period, of which 891 were excluded based on the criteria described above. Of the 545 cases that met inclusion criteria, 296 (54.3%) were females and 249 (45.7%) were males. There were 188 cases (34.5%) with a documented serum sodium, including 73/188 (38.8%) with hyponatremia (Na < 130 mmol/L). Of the 73 subjects with hyponatremia, there were 55 (75.3%) females and 18 (24.7%) males; of the 115 non-hyponatremic subjects 50 (43.5%) were female and 65 (56.5%) male. There was no observed gender difference in the ascertainment of serum sodium levels. Among patients with a documented serum sodium level, female gender was 3 associated with increased odds of hyponatremia by univariate analysis (OR 3.97; 95% CI 2.08 -7.59). Conclusion:Female gender was associated with increased odds of hyponatremia among persons with Ecstasy intoxication and a documented serum sodium level reported to CPCS from 2000 -2005. Multiple potential study confounders, including spectrum bias, incomplete laboratory data and individual differences in subject characteristics prevent determination of causality regarding gender differences in the incidence of Ecstasy-associated hyponatremia and its complications.