The objectives of this study are to evaluate the patients who have sinus headaches, either self-ascribed or physician-diagnosed, and to determinate the response to sodium valproate in a prophylactic treatment of the patients without positive sino-nasal findings. "Sinus headache" is a patient's complaint or physician-diagnosis that can have a variety of underlying causes. The patients are often treated with multiple courses of antibiotics and occasionally undergo a sinus surgery, often with little or no relief of their symptoms. One hundred and four patients with "sinus headaches" were evaluated prospectively. The patients with a normal rigid nasal endoscopy and a paranasal sinus computed tomography scan were treated with sodium valproate as a prophylactic treatment. After a 3-month follow-up, the patients' response to the treatment was evaluated. Seventy-two patients (69.2%) did not have any positive sino-nasal findings in the nasal endoscopy and the computed tomography scanning. The response rate to the treatment for these patients was as follows: significant improvement in 44 patients (61.1%), partial response (9.7%), no response (15.3%), and ten patients (13.9%) withdrew or failed to follow-up. According to Wilcoxon test, the patients' response rate to sodium valproate was statistically significant (P = 0.001). In conclusion, a majority of "sinus headache" patients do not show any positive sino-nasal pathologic finding. Therefore, we have to consider migraine headache as a considerable cause and sodium valproate as an effective conservative treatment.