Chronic subdural hematoma as a complication of lumbar drain placement for the management of iatrogenic cerebrospinal fluid (CSF) leak has not been previously documented in the literature. We describe such a case in a 69-year-old man who presented with right nasal obstruction secondary to an inverted papilloma involving the paranasal sinuses. The patient underwent endoscopic sinus surgery, which included a medial maxillectomy. Surgery was complicated by a small CSF leak, which was repaired intraoperatively. Five days later, the patient experienced CSF rhinorrhea, and a lumbar drain was inserted. He developed overdrainage symptoms but was well when he was discharged. However, 22 days later he returned with right hemiparesis. Computed tomography of the brain showed a left frontoparietal subdural hematoma with a mass effect. The neurosurgical team performed an emergency drainage procedure, and the patient experienced a complete neurologic recovery. We discuss the pitfalls of lumbar drainage, the possible pathophysiology of overdrainage, and the lessons learned from this case.