Background: Several approaches to the ACL attachment and drilling methods exist, with little evidence of which method is the best. Hypothesis: The “Retrodrill” or an “inside-out” drilling technique result in uniform intra-articular tunnel mouths compared with standard “outside-in” conventional ACL drill bits. Study Design: Controlled laboratory study. Methods: Sixteen cadaveric knees were divided into Anterograde (A) and retrograde “Retrodrill” (R) groups and ACL tunnels drilled. The femoral tunnel mouths were moulded using PMMA, then Nylon rods of identical diameter containing 1mm diameter K-wires were inserted into the tunnels and AP and lateral X-rays taken. Matching laboratory experiments utilised 10 pig femurs and synthetic bone. Results: In group A, the mean difference between tunnels and their mouths was 1.6±0.5mm, compared with 0.3±0.2mm for Group R (p < 0.001). The mean femoral tunnel angulation in the sagittal plane for Group A was 45±10 degrees and 78±14 degrees for Group R (p< 0.001); and 30±12 degrees, and 71±12 degrees (p< 0.001) in the coronal plane respectively. There were similar ACL attachment hit rates from the groups. In porcine bone, tunnel mouth widening in the anterograde tibial group was 0.7±0.4mm, 0.04mm±0.1mm for the anterograde femoral group, and 0.06±0.1mm for the retrograde group (p< 0.001). In synthetic bone, the difference between the tunnels and their mouths was 0.8±0.8mm, 0.2±0.1mm and 0.1±0.1mm (p< 0.001) respectively. Conclusions: The “Retrodrill”, or an inside out antero-grade drilling technique, produced a more uniform tunnel with no difference in ACL attachment hits