Objective: In this study, we determine the effectiveness and adverse effects of sumatriptan when used in the emergency department (ED) as a first-line treatment for benign undifferentiated headaches, and determine if the International Headache Society (IHS) classification of migraine, probable migraine, or tension-type headache has any effect on the effectiveness of the treatment. We hypothesize that there is no difference in the effectiveness of pain relief or frequency and severity of adverse effects between patients with migraine, probable migraine, or tension-type headaches when treated with sumatriptan. Methods: This was a prospective observational study of adult ED patients undergoing treatment for primary headaches (ie, patients in whom head trauma, vascular disorders, infection, or disorders of facial or cranial structures have been clinically excluded). Other exclusions were renal impairment, hepatic impairment, and risk factors for coronary artery disease. Consenting patients then were asked to complete a 100-mm visual analog scale (VAS) representing their perceived pain, after which they were interviewed by a research assistant who completed a headache diagnosis worksheet, which differentiates the headache by IHS criteria. The patient repeated the VAS score at 30 and 60 minutes. Data were analyzed using descriptive statistics and χ2 tests. Results: One hundred forty-seven patients were enrolled: 84 (57.1%) patients with migraine headache, 45 (30.7%) with a probable migraine headache, and 18 (12.2%) with a tension headache. A 50% reduction in VAS scores 60 minutes postdose was seen in 87 (59%) of 147 patients; 50 (60%) of 84 of migraine patients, 25 (56%) of 45 of probable migraine patients, and 12 (67%) of 18 tension patients (P = .72). There were no serious adverse events reported. Forty-seven patients (32%) received rescue medications after the 60-minute VAS score: 29 (34.5%) patients in the migraine group, 15 (33.3%) patients in the probable migraine group, and 3 (15.8%) patients in the tension-type headache group (P = .26). Conclusions: Most of the patients presenting with primary headaches had migraine or probable migraine headaches. There was no difference in sumatriptan's effectiveness based on the classification of the headache using IHS criteria.