Although ganglionic cysts located at the hip joint are described infrequently, those found in this region are usually small, deep-seated, and asymptomatic. Occasionally, however, a large ganglionic cyst of the hip area is observed that becomes symptomatic following compression of adjacent neurovascular bundles. In this report, the authors describe a 51-year-old man with symptoms of sciatica caused by a giant ganglionic cyst of the posterior hip joint. Because of its intermuscular location, the cyst was not palpable, and was probably misdiagnosed previously as a herniated disc of the lumbar spine. After resection of the cyst, the patient's symptoms resolved completely. This case highlights the importance of a detailed clinical examination for patients with multiple degenerative joint diseases.