To refine the assessment of over-the-counter analgesic agents in the treatment of muscle-contraction headache, we designed a single-dose model with attention to specific methodologic features and two relevant assessments--the percentage of subjects who achieve complete relief and the time until pain is no longer experienced. Subjects were randomly assigned to receive a single dose of 1000 mg acetaminophen, 1000 mg aspirin with 64 mg caffeine, or placebo. Under double-blind conditions, subjects rated headache pain intensity and relief over 4 hours and provided a Comparative Evaluation at the end of the trial. Both active agents were significantly distinguished from placebo on the time-point analyses (p less than 0.05) and summary end point measurements (sum of pain intensity difference [SPID], total of pain relief, percentage of patients with complete relief, percentage of treatment failures, and the Comparative Evaluation), as well as causing a faster elimination of headache (p less than 0.05). The aspirin-caffeine combination was rated higher than acetaminophen on all summary measurements, particularly SPID (p less than 0.05), with significantly more patients obtaining complete relief with aspirin-caffeine (p less than 0.01) than with acetaminophen. We conclude that this headache pain model can be used to demonstrate the efficacy of over-the-counter analgesic agents and to assess their relative efficacy.