This article is concerned with the study of the effect of several variables, principally that of cerebrospinal fluid drainage, on the incidence of neurologic deficit in a prospective randomized series of patients with extensive aneurysms of the descending thoracic and abdominal aorta (thoracoabdominal type I and II). Forty-six patients had cerebrospinal fluid drainage, and 52 were controls, with a total of 98 available for study. Cerebrospinal fluid pressure was continuously monitored in the former group and pressure maintained less than or equal to 10 mm Hg in 20, less than or equal to 15 mm Hg in 20, and greater than 15 mm Hg in 6 patients during period of aortic clamping. The method of treatment including reattachment of intercostal and lumbar arteries (p = 0.2), temporary atriofemoral bypass during aortic occlusion (p = 0.3), and spinal fluid drainage (p = 0.8) were not statistically significant in reducing the incidence of neurologic deficits. Thus cerebrospinal fluid drainage as we used it, was not beneficial in preventing paraplegia. On appropriate statistical analysis we found that the only significant predictor of delayed deficits was postoperative hypotension (p = 0.006).