Chronic subdural hematoma (CSDH) tends to occur in elderly patients with a history of mild head injury at a few months prior to the onset of symptoms. Intracranial arachnoid cyst is believed to be congenital and sometimes becomes symptomatic in pediatric patients. These two distinct clinical entities sporadically occur in the same young patient. Twelve of 541 cases of CSDH surgically treated in our institution had associated arachnoid cyst. The clinical and radiological characteristics of the cases of CSDH associated with arachnoid cyst were retrospectively analyzed and compared with those of CSDH without arachnoid cyst. Arachnoid cysts were located in the middle fossa (eight cases), convexity (two cases), and posterior fossa (two cases). Three cysts were less than 20 mm in diameter. The 12 patients with CSDH and arachnoid cyst (mean age 27.8 +/- 19.7 years) were significantly younger (p < 0.001) than the patients with CSDH without arachnoid cyst (69.5 +/- 13.7 years). Five of the 12 patients were pediatric cases (< 15 years old). The clinical symptoms were also significantly different. The most frequent symptom was headache followed by vomiting in the patients with arachnoid cyst, while gait disturbance and hemiparesis predominated in patients without arachnoid cyst. Hematoma evacuation through burr holes improved the symptoms in all patients with arachnoid cyst. We conclude that even a small arachnoid cyst can be a risk factor for CSDH after mild head injury in young patients and symptoms of increased intracranial pressure are common. Hematoma evacuation is adequate at first operation. If the preoperative symptoms persist, additional arachnoid cyst surgery should be considered. The present results also suggest that CSDH formation may be preceded by subdural hygroma caused by the rupture of arachnoid cyst.