Sixty-five patients who underwent wide laminectomy, Cotrel-Dubousset instrumentation, and fusion for lumbar degenerative disorders were reviewed radiographically to investigate the incidence and contributing factors of the postfusion instability at the adjacent segments immediately above or below the level of fusion. Thirty-four were men and 31 were women. The mean age was 55.8 years. The average follow-up time was 39 months. Postfusion instability was studied at a total of 107 adjacent segments in 65 patients. The incidence of postfusion instability noted at final follow-up was 24.6% (15 of 61 patients). The incidence was significantly more often observed in the adjacent segments above the fusion than below the fusion, at rates of 25.5 and 2.6%, respectively. The most common instability was posterior translation (9 of 15 patients). Regarding contributing factors for instability at the adjacent segment above the fusion levels, the age of patients was the most significant factor. The incidence was 36.7% (11 of 30 patients) in older patients (> 55 years old) and 12% (3 of 25 patients) in younger patients (< 55 years old). In four patients with a preoperative of > 3 mm anterior translation, instability progressed further postoperatively. To prevent postoperative instability, attention must be paid especially above the fusion levels of the elderly and the preoperative minimal anterior translation.