Ganglion cyst of the spinal facet joint is rare. It is believed that ganglion cysts on the dorsal aspect of the facet joints are asymptomatic and therefore are not clinically appreciated. But we have experienced a patient who had severe low-back and buttock pain and diagnosed as ganglion cyst located on the dorsal aspect of the right side facet joint between the fourth and fifth lumbar vertebrae. The treatment was surgical excision of the cyst and resulted in a satisfactory recovery. The differential diagnosis involving herniated lumbar intervertebral disc should not exclude ganglion cyst of the dorsal aspect of the lumbar facet joint.