A 15-year-old girl developed a rare case of subdural hygroma after foramen magnum decompression for Chiari type 1 malformation manifesting as rapid symptom deterioration around 10 days after uncomplicated operation with uneventful immediate postoperative course. Progressive enlargement of the subdural hygroma in both supra- and infratentorial spaces was followed by the development of hydrocephalus. Syringomyelia improved shortly after the first operation but then deteriorated with massive presyrinx formation. Reoperation with wide opening of the arachnoid membrane lead to a rapid resolution of the hydrocephalus and the presyrinx. The present case shows that wide opening of the arachnoid membrane is an effective therapeutic option.