Rehabilitation of patients with anterior cruciate ligament injury is a topic of interest among clinicians. Although numerous studies report the deficits after anterior cruciate ligament reconstruction, there are no data available on the changes in strength and functional performance before and 6 months after reconstruction surgery. This is when most patients are returning to sports activities. In the current study 31 patients with complete unilateral anterior cruciate ligament ruptures followed a controlled rehabilitation program emphasizing early range of motion after surgery and quadriceps strengthening before and after undergoing bone-patellar tendon-bone reconstruction. Strength of the hamstrings and quadriceps and five functional activities were assessed at 1 week before and 6 months after surgery. Preoperative strength assessments using a Cybex II dynamometer showed a 12% quadriceps deficit at 60 degrees per second and 9% deficit at 120 degrees per second with no hamstring deficit in the injured leg compared with the uninjured leg. Despite intensive quadriceps training, the postoperative quadriceps deficit increased to 28% at 60 degrees per second and 22% at 120 degrees per second in comparison with the uninjured leg. There was no change in hamstring strength in the injured leg. Most functional measures improved after surgery. The shuttle run improved by 10%, the side step test results improved by 17%, and the carioca test results improved by 23%. No significant difference was found in the results of the hop test. This study highlights the extent of the quadriceps deficits despite functional improvement and the difficulty in restoring quadriceps strength after bone-patellar tendon-bone reconstruction. It also questions the relationship of current functional tests to quadriceps strength and the role of maximum strength testing as a predictor of function and of safe sporting performance.