Tension headache (TH) belongs to the most frequent type of idiopathic headaches. Depression and dysfunction of serotonin metabolism play a key role in the pathogenesis of TH. The study of headache intensity, psychometric testing and determination of blood serotonin (S) concentration using the method of enzyme immunoassay before and after treatment with antidepressant prodep were performed in 140 patients with TH. All the patients had moderate pain syndrome, depression and anxiety of high and moderate severity that were in inverse correlation with blood S concentration. Association of high indices of state (46.81 +/- 8.86 scores) and trait (54.2 +/- 7.64) anxiety and depression (22.4 +/- 6.84, the Beck depression scale) with the marked decrease of blood S (119.38 +/- 51.2 ng/ml) was found in 96 patients of this group. The significant reduction of headache intensity and depression and the increase of serum S after prodep therapy was observed. Therefore, a serum S level may be regarded as a marker of pain intensity and depression level and an objective index of antidepressants efficacy.