Vacuum phenomenon is a commonly observed radiological entity in the degenerated intervertebral discs of the lumbar spine in the elderly population. The entity is frequently asymptomatic. Although disc herniation containing gas (DH-CoG) is commonly associated with the vacuum phenomenon, DH-CoG associated with clinical symptoms is a rare condition. There are very few reports which have histologically demonstrated the existence of the gas itself within DH-CoG. Herein, we report a rare case of a 65-year-old female with symptomatic DH-CoG at L5/S1. The patient was admitted to our hospital with a one-month history of pain in the left buttock and leg in addition to neurogenic claudication. Roentgenograms illustrated a degenerative lumbar spine with the vacuum phenomenon at the L5/S1 disc space. Computed tomography showed a round and low-density lesion within the spinal canal at left L5/S1. Additionally, a lesion characterized by an iso- and partially hypointense signal on T1 and hypointense signal on T2 was detected in magnetic resonance imaging (MRI) by the spin-echo method. The decision for posterior lumbar interbody fusion surgery using pedicle screws was made as the symptoms had not responded to the conservative treatment. After a degenerated prolapsed nucleus was carefully extracted, the specimen was sent to the laboratory for histopathological analysis. The prolapsed nucleus of DH-CoG histologically showed many small vacuoles containing degenerated mucopolysaccharides. The left leg pain drastically resolved on the first post-operative day, and no recurrence had been observed. Degenerated mucopolysaccharide may be a precursor of nitrogen or “the gas itself” in DH-CoG. Surgical intervention for DH-CoG should be considered if conservative treatment fails.