BACKGROUND: Radiographic bone tunnel enlargement after anterior cruciate ligament reconstruction is frequently reported. The error associated with measuring tunnels on radiographs has not been established and may be substantial. HYPOTHESIS: Measurement error influences interpretation of bone tunnel enlargement. STUDY DESIGN: Cohort study (Diagnosis); Level of evidence, 2. METHODS: Radiographs and clinical outcomes were obtained from 76 subjects who had undergone hamstring tendon anterior cruciate ligament reconstruction. Tunnel enlargement was determined by measuring the widths of the femoral and tibial tunnels with a digital caliper in lateral and anteroposterior radiographs. Measurements were corrected for magnification; the percentage change in tunnel widths was recorded relative to the diameters drilled at surgery. One examiner measured all radiographs and repeated the procedure for 20 randomly selected radiographs 12 months later. A second examiner measured the same 20 radiographs. Intraclass correlation coefficients and 95% confidence intervals for intrarater and interrater measurement errors were calculated. Confidence intervals were used to divide subjects into 2 groups according to whether bone tunnel enlargement was within (unenlarged tunnel group) or greater than (enlarged tunnel group) the 95% confidence interval for measurement error; clinical comparisons between these groups were made for each view of tibial and femoral tunnels. RESULTS: Measurement errors for tunnel enlargement ranged from 17% to 26% for intrarater measurements and 24% to 38% for interrater measurements. For the lateral view of the tibial tunnel, there was a significant positive association between tunnel enlargement and anterior knee laxity. There was no relationship between femoral tunnel enlargement and anterior knee laxity. CONCLUSION: There was considerable error associated with bone tunnel measurement. A relationship between tibial tunnel enlargement on the lateral radiograph and anterior knee laxity was evident once enlargement was defined as a change that exceeded measurement error. CLINICAL RELEVANCE: Measurement of bone tunnels on radiographs is useful for interpreting large changes; caution should be used when small changes are observed.