METHODS A prospective clinical study using venography to evaluate deep venous thrombosis after posterior spinal surgery. OBJECTIVE To demonstrate the prevalence of deep venous thrombosis after posterior spinal surgery with no prophylaxis. BACKGROUND There have been few studies about the occurrence of deep venous thrombosis after spinal surgery in which venography was used for screening. METHODS Of the enrolled 134 patients undergoing posterior spinal surgery, 110 (82.1%) were examined with complete surveillance for deep venous thrombosis by venography. There were 64 males and 46 females. The average age at operation was 59.0 years (range, 14-86 years). The levels of the operation were cervical in 54, thoracic in 7, and lumbar in 49. All procedures were performed with patients under general anesthesia. Neither mechanical methods nor anticoagulation medications were used for prophylaxis against thromboembolism. Bilateral ascending venography was performed within 14 days after surgery. RESULTS There were no patients with clinical signs of deep venous thrombosis and pulmonary embolism. However, 17 patients (15.5%) showed venographic evidence of deep venous thrombosis, of whom 16 had distal thrombi, and only one had a proximal thrombus. Deep venous thrombosis was venographically evident in 3 (5.6%) of 54 patients who underwent cervical procedures, and it was evident in 13 (26.5%) of 49 patients who underwent lumbar procedures. This difference was statistical significant (chi2 test, P = 0.003). Statistical comparison between patients who did and did not have deep venous thrombosis showed that age was statistically significant (Mann-Whitney test; P < 0.05). CONCLUSIONS The prevalence of deep venous thrombosis after posterior spinal surgery is higher than generally recognized. Therefore, further study is necessary to clarify the appropriate method for screening and the effect of prophylaxis against thromboembolism after spinal surgery.