STUDY OBJECTIVE: To determine the efficacy of dantrolene sodium in the treatment of heatstroke. DESIGN: Randomized, double-blind, placebo-controlled trial. SETTING: Heatstroke center in Makkah, Saudi Arabia. PATIENTS: Fifty-two adult patients with heatstroke. INTERVENTIONS: Patients were assigned to receive either dantrolene sodium (2 mg/kg body weight iv) or placebo. Conventional cooling therapy was initiated in all. MEASUREMENTS AND MAIN RESULTS: There was no significant difference in the mean cooling times for the treatment and control groups (67.9 vs. 69 min). There was only one death in the control group. Complications were seen in six (23%) patients receiving dantrolene sodium and seven (27%) patients receiving placebo; the difference was not statistically significant. There was no significant difference in the mean number of hospital days (4.7 +/- 2.0 vs. 2.9 +/- 0.9 days). CONCLUSION: Treatment with dantrolene sodium at the dose used, did not prove beneficial to patients with heatstroke.