Sixteen patients suffering from hemicranial attacks are reported. After many years of unsuccessful conservative treatment (mean = 12.4 years), the patients were treated surgically with good results. The radiological or electrophysiological examinations were non-specific or negative. Only vasoactive tests (provoking or relieving pain) or local anesthesia proved helpful in diagnosing and localizing the origin of pain. Intraoperatively, hemicranial attacks were found to be caused by vascular irritation or compression of the cervical nerve root C2. After decompression (n = 6) or dissection (n = 10) of the nerve root and the ganglion, 12 patients were relieved of their pain, 2 had improved relatively, 1 showed only a slight improvement, and in 1 patient no cause was found and no improvement was achieved. Two patients suffered recurrence of pain postoperatively; one had no further complaints after root extirpation following percutaneous thermorhizotomy. Electron microscopic examination of the nerve root and its ganglion revealed focal morphological changes, including proliferation of connective tissue in the endoneurium and the ganglion itself, the formation of onion-bulb-like structures around single axons, discrete signs of myelin damage and axonal degeneration. These morphological changes are possibly the result of a chronic vascular compression.