A 25-year-old man presented with left lumboischialgia refractory to medical treatment. Neurological examination revealed L5 and S1 radiculopathy which rapidly worsened over a short period. Magnetic resonance imaging demonstrated disk bulging with a discal cyst at the L4-5 intervertebral space and disk herniation at the L5-S1 intervertebral space. Computed tomography showed osteolytic change of the L5 vertebral body adjacent to the cyst. Resection of the cyst and removal of the herniated disk were performed following fenestration of the L4-5 and L5-S1 interlaminar spaces. Bloody serous fluid followed by clear serous fluid was recognized during the aspiration and partial resection of the cyst at the L4-5 level. Histological examination demonstrated a cyst wall consisting of fibrous connective tissue without a single-layer lining of cells, and fibrin deposits. The patient's symptoms disappeared immediately after the operation. This osteolytic lumbar discal cyst possibly occurred subsequent to hemorrhage from the epidural venous plexus following intervertebral disk injury, hematoma encapsulation by connective fibrous tissue, and cyst wall formation in reaction to the disk injury and hemorrhage. The cyst may have enlarged due to the inflow of the serous fluid from the water-containing degenerated disk.