92 patients (between the age of 18 and 64), with degenerative disc disease undergoing lumbar spinal fusion (LSF), were randomly assigned to groups that started rehabilitation at either 6 or 12 weeks after surgery. The results showed that there was no significant difference in physical performance (6-min walking test and Astrand Fitness-tests) between the two rehabilitation groups following treatment (1 year after surgery). When comparing the physical performance tests to questionnaire outcome measures, a significant correlation was found between 6-min walking test and questionnaire outcome measures. No correlation was found between the outcome measures and the Astrand Fitness -test.