Secondary piriformis syndrome may develop under several conditions. We report on the method and usefulness of arthroscopic treatment of the piriformis muscle and benign perineural cyst of the sciatic nerve. Arthroscopic assessment was performed to a perineural cyst on the sciatic nerve seen through EMG and hip MRI. Through the posterior and posteroinferior portal, the lesion below the piriformis muscle was confirmed following release of the tendon. The incision and drainage of the perineural cyst was done so as to achieve decompression. The excision of the lesion was not carried out so as to avoid injury to the sciatic nerve. Symptoms did not recur during 10 months of follow up appointments.