Cervicogenic headache (CEH) is a unilateral headache that can be provoked by neck movement, awkward head positions or pressure on tender points in the neck. The mechanisms underlying the stimulation of pain in CEH are not clearly known. In this study, we measured serum nitrate and nitrite levels as an index of nitric oxide (NO) activity in 15 patients with CEH during headache and headache-free periods and in 15 healthy controls. Total nitrate+nitrite levels were found to be higher in CEH patients during headache periods than in healthy controls (20.7±3.8 μmol/l vs 14.4±3.6 μmol/l, p<0.001), but not in CEH patients during headache-free periods (16.1±2.2 μmol/l) compared with the controls (p>0.05). In the patients with CEH, serum total nitrate+nitrite levels were found to be higher during headache periods than during headache-free periods (p=0.001). It can thus be hypothesized that the changes observed are a cause of the attack rather than a consequence of the disease process.