In this multicenter, double-blind, parallel study, patients with muscle contraction headache were randomized to treatment with either two naproxen sodium 275 mg capsules per headache or matching placebo. Of a total of 161 patients enrolled, 137 received trial medication and 134 (63 taking naproxen sodium and 71 taking placebo) were valid for efficacy analysis. During four headache episodes, patients recorded, on a visual analog scale, pain intensity just before the dose of trial medication (baseline) and at1, 2, and 4 hours after this dose. For each patient, two additional indices were derived from these data: Pain Intensity Difference (PID), the difference between intensity of pain at baseline and intensity of pain recorded at any time after treatment began; and Summed Pain Intensity Difference (SPID), the sum of the PIDs at a given hour. Results significantly favored naproxen sodium over placebo (p<0.05) at all times. Naproxen sodium was found to be well tolerated and was more efficacious than placebo in the treatment of muscle contraction headache.