Endoscopic third ventriculostomy (ETV) is a safe treatment option for chronic obstructive hydrocephalus. However, we encountered a case of chronic subdural hematoma (CSDH) with bilateral large hematoma volumes after ETV for chronic obstructive hydrocephalus. We herein report a rare complication of ETV. The patient was a 53-year-old woman who had been diagnosed with asymptomatic ventricular enlargement with aqueductal stenosis 5 years previously. However, over the course of 5 years, her gait and cognitive function gradually declined. ETV was administered to relieve symptoms. Head Magnetic resonance imaging performed 1 week after ETV indicated bilateral subdural hygroma. Three weeks after ETV, she presented with headache and left incomplete paralysis, and head Computed tomography (CT) demonstrated bilateral CSDH with a large volume hematoma. Burr-hole evacuation and drainage of the bilateral CSDH were performed, after which the symptoms resolved. However, 7 weeks after ETV, she again presented with headache and incomplete right paralysis, and CT revealed bilateral CSDH re-enlargement. After the second burr-hole evacuation and drainage of bilateral CSDH, her symptoms resolved. The bilateral CSDH continued to shrink following the second hematoma evacuation surgery and completely disappeared on CT scan performed 3 months after ETV. Ventricular enlargement due to chronic obstructive hydrocephalus stretches the brain mantle for several years. This long-term stretching may have diminished the brain compliance and led to the development, growth, and recurrence of CSDH. In ETV for chronic obstructive hydrocephalus, surgeons should consider the risk of postoperative CSDH with a high hematoma volume and tendency to recur.