1. The complications after low-back fusion in two series of 500 cases each, one decade apart, were analyzed and compared.

2. The most common complication was urinary retention. Prophylactic sulfonamide therapy after catheterization prevented subsequent urinary tract infection.

3. The over-all incidence of thromboembolic complications was 3.2 per cent. The incidence decreased from 4.2 in the first to 2.2 per cent in the second series concomitantly with, and probably as a result of, earlier walking.

4. The immediate institution of anticoagulant therapy following the diagnosis of thromboembolism resulted in complete recovery in the thirty-two cases in the combined series.

5. The incidence of thromboembolic complications was found to be slightly higher in older patients and in those with prior history of peripheral vascular disease.

6. Peripheral phlebothrombosis developed on the side from which the iliaccrest graft was taken in 67 per cent of cases.

7. Wound infection as a complication of spine fusion occurred in 3.4 per cent of our patients.

8. Increased operating time and early ambulation after spine fusion did not appear to increase the risk of infection.

9. Postoperative antibiotic prophylaxis did not decrease the incidence of infection and may have increased the risk.

10. Pseudarthrosis developed in 15.1 per cent of the 430 cases followed, with the lowest rates following fusion by the H-graft and lateral-element techniques.

11. Pseudarthrosis developed in only 10.3 per cent of the multiple-joint fusions in which the lateral-element technique was employed.

12. The incidence of pseudarthrosis increased as the number of vertebrae in the fusion mass increased.

13. Earlier walking did not increase the incidence of pseudarthrosis.