Headache is a major symptom in chronic subdural hematoma (CSDH) patients. However, some CSDH patients do not complain headache although the hematoma is thick with definite midline shift. This clinical study was performed to identify the mechanism of headache in CSDH patients. We compiled clinical data of 1080 surgically treated CSDH patients (711 males and 369 females), and in 54 cases, the pressure of hematoma was measured during burr hole surgery using a glass-stick manometer. Headache was recognized in 22.6% of patients, while nausea or vomit suggesting increased intracranial pressure was detected in only 3.0%. Ophthalmological examination was performed in 238 patients, and papilledema was identified in only one patient (0.4%). The mean age of patients with headache (59.8 ± 16.9 years) was significantly younger than that of those without (75.7 ± 11.2 years) (P &lt
 0.01). In 54 cases, the mean hematoma pressure was not significantly different between patients with (17.1 ± 6.2 mmH2O) and without (18.4 ± 7.2 mmH2O) headache (P &gt
 0.10). Hematoma thickness was significantly greater in patients without headache (P &lt
 0.01), but the ratio of midline shift to hematoma thickness was significantly greater in patients with headache (P &lt
 0.01). In our results, the status of increased intracranial pressure was rare in CSDH patients, and high hematoma pressure was not a cause of headache. Midline shift was the most influenced factor for headache in our study, and based on the results, the authors consider that the potential cause of headache in CSDH might be stretching or twisting of the pain-sensitive meninges and meningeal arteries or veins.