Two cases of sciatica secondary to nerve root compression by a "synovial cyst" of a zygapophyseal joint are described. In light of these 2 cases and on reviewing the literature, it appears that zygapophyseal joint osteoarthritis with degenerative (or articular) spondylolisthesis can be a predisposing factor to the formation of such synovial expansions and, consequently, the cause of nerve root compression. The coexistence of a lumbar degenerative spondylolisthesis with a radicular syndrome should therefore encourage early investigation by computed tomography scan, so that conservative treatment would not be unnecessarily prolonged.The most common cause of sciatic pain is nerve root compression secondary to disc herniation, but nerve root compression can be caused by other structures, and computed tomography (CT) scanning may reveal, and can often differentiate, them. It appears that capsulosynovial expansions of the zygapophyseal joints occur with relative frequency. Most often, the expansions are related to zygapophyseal joint osteoarthritis (OA), which in some cases, is also responsible for an anterior displacement of the upper vertebrae (degenerative spondylolisthesis). We describe here 2 typical cases, with documentation by CT scan, neurosurgery, and histologic examination.CASE REPORTS Patient 1. The patient, a 77-year-old female homemaker, was hospitalized because of a 1-month history of left-sided lumbosciatic pain that had no precise radicular radiation. The pain was not aggravated by coughing or defecating and was not accompanied by paresthesias. There was no history of preceding tpauma. Since 1974, this patient had been treated with levodopa because of Parkinson's disease. In 1982, she experienced a transitory cerebrovascular accident.Physical examination findings indicated that the patient's general state of health was good. She presented with an acute vertebral syndrome, mostly on the left side. Pain limited dorsolumbar mobility in all 4 directions, particularly in extension, which was almost impossible. Results of the neurologic examination were entirely normal, including the absence of Las8gue's sign. The laboratory test results were normal.Standard films of the lumbar spine showed a first-degree degenerative spondylolisthesis of L4 on L5, in connection with a severe L4-L5 zygapophyseal joint OA. In addition, there was advanced disc degeneration of L5-S 1 , showing a "vacuum phenomenon" (Figure 1).