Purpose To investigate progressive tunnel widening and its correlation with postoperative outcomes after anterior cruciate ligament (ACL) reconstruction using allografts. Methods Sixty-ive patients who underwent ACL reconstruction using a tibialis anterior allograft between 2015 and 2017 were enrolled. Femoral and tibial tunnel widths were measured on anteroposterior (AP) and lateral radiographs immediately and at 3, 6, 12, and 24 months postoperatively. Average femoral and tibial tunnel widths in AP and lateral views were calculated at three diferent measurement points. Tunnel widening was calculated as the diference in tunnel width immediately and 2 years postoperatively. The correlation between tunnel widening and the postoperative results was analysed. Results Tunnel width changes between immediate and 2 years postoperatively were as follows, in AP and lateral views, respectively: femur, 3.0 mm ± 1.5 mm and 2.4 mm ± 1.4 mm; and tibia, 2.8 mm ± 1.4 mm and 2.9 mm ± 1.5 mm. Femoral tunnel widths signiicantly increased until 1 year, but not from 1 to 2 years postoperatively. Tibial tunnel width signiicantly increased until 2 years postoperatively. In all tunnels, the increments in tunnel widening decreased over time. Increased knee laxity signiicantly correlated with greater femoral tunnel widening in AP (r = 0.346, P = 0.006) and lateral views (r = 0.261, P = 0.049). Conclusion Femoral tunnel widths gradually increased until 1 year postoperatively, and tibial tunnel widths increased until 2 years after ACL reconstruction with allografts. The tunnel widening rate gradually decreased over time. Femoral tunnel widening of 3.7 mm and 3.2 mm on AP and lateral views, respectively, were the cut-of values for postoperative knee laxity. Level of evidence Level III.