Radiographic increase in the size of tibial and femoral tunnels has been observed. This retrospective study compared tibial tunnel diameter in 56 autograft and 87 allograft patellar tendon bone-tendon-bone anterior cruciate ligament replacements whose observed tunnel changes were correlated with clinical results at 1 year postoperatively. Tibial tunnel sclerotic margins were measured approximately 1 cm below the joint line. Exact tunnel dimension was calculated by using a magnification factor determined by the interference screw of known diameter within the same tunnel. Average allograft tunnel enlargement was 1.2 mm (-2.5 to 6.0) compared with the autograft tunnel enlargement of 0.26 mm (-2.5 to 2.7); the difference was significant (P > 0.0002). No significant difference was seen in KT-1000 arthrometer measurements between autograft or allograft groups, and no correlation was seen between increased tunnel size and clinical outcome as determined by the modified Hughston knee evaluation system. Tunnel measurement reproducibility was confirmed by independent repeated measurements. The significance of this tunnel enlargement is unknown and does not appear to adversely affect clinical outcome of allograft utilization. Possible explanations include an immune response with resorption, stress shielding proximal to the interference screw resulting in resorption, or an inflammatory response by synovium in the tunnel.