Tibial tunnel enlargement is a common occurrence following multiple techniques of anterior cruciate ligament reconstruction. The etiology is felt to be multifactorial: biological and mechanical. Biologic mechanisms include the presence and persistence of synovial fluid and catabolic cytokines in the tibial tunnel. We hypothesized that retaining the native ACL stump may provide a “stopper” effect, preventing synovial fluid leakage into the tibial tunnel, and this assists in preventing tunnel enlargement from occurring. The purpose of this randomized prospective MRI study was to determine the presence of synovial fluid in the tibial tunnel following two different surgical techniques of ACL reconstruction using a quadrupled hamstring autograft. Materials and Methods: We randomized 20 patients having ACL reconstruction with a quadrupled hamstring autograft into two groups. Group A consisted of 11 patients who retained the native ACL stump. Group B consisted of 9 patients who had the native ACL stump completely resected. All patients underwent MRI scans at an average of 45 days postoperatively in order to determine tunnel measurements and to evaluate for the presence of synovial fluid in the tunnel. Results: Four patients in each group demonstrated fluid in the tibial tunnel on a postoperative MRI. Ten of the 20 patients had 2 postoperative MRIs at an average of 34 and 68 days postoperatively. None of these patients demonstrated fluid in the tibial tunnel on the first MRI, but three demonstrated fluid in the tibial tunnel on the second MRI. There was an increase in diameter of the tibial tunnel in 5 patients between the first and second MRI. Of the 5 which demonstrated increasing tunnel diameter, 2 had fluid present in the tunnel, both of which were in group B. Discussion: The cause of tunnel enlargement following ACL reconstruction is multifactorial. Synovial fluid presence within the tunnel may be associated with enlargement. In our study, native ACL stump preservation did not appear to limit synovial fluid leakage into the tunnel.