PURPOSE: The purpose of this study was to compare measurements of bone tunnel enlargement after anterior cruciate ligament (ACL) reconstruction using digital plain radiography and computed tomography (CT). TYPE OF STUDY: Within-subjects comparison of 2 imaging techniques. METHODS: Twenty-two patients who had undergone primary ACL reconstruction had anteroposterior and lateral digital plain radiographs and a CT scan of their operated knee 12 months after surgery. Using digital calipers, the sclerotic margins of each tunnel were measured at the widest point in each plane. Measurements were corrected for magnification and expressed as a percentage change in tunnel width compared with the size of the drill bit used at surgery. RESULTS: In all but 1 instance, bone tunnels seen on CT were also seen on the corresponding digital radiograph. In the single incidence in which a bone tunnel was identified on the CT scan but not on the radiograph, the tunnel had the smallest diameter of the group and had decreased since surgery by 30%. There was no difference between the group-averaged CT and radiographic measurements for the tibia, but femoral tunnel radiographic measurements were clearly larger than CT, particularly for the anteroposterior view (P < .001). Moderate discrepancy was noted between individual radiograph and CT measurements. CONCLUSIONS: Digital plain radiography appears to be satisfactory for detecting bone tunnel enlargement following ACL reconstruction. Thus it may offer a time- and cost-effective means for monitoring this phenomenon. LEVEL OF EVIDENCE: Level I.