OBJECTIVE There is limited literature investigating the reliability of magnetic resonance-based assessments of labral size. The goal of this study was to validate the reliability of magnetic resonance arthrography-based labral size measurements with intra-operative arthroscopic measurements. METHODS Patients undergoing hip arthroscopy for femoroacetabular impingement and labral tears were prospectively enrolled. Preoperative magnetic resonance arthrograms were used to determine labral size at the anterior-superior portion (zone 2), mid-superior portion (zone 3), and posterior-superior portion (zone 4). Intra-operative labral widths were measured at the same anatomical zones of the acetabulum using an arthroscopic probe. Mean labral size was determined for each location and a Pearson correlation was used to determine the correlation between imaging-based measurements and intra-operative measurements. RESULTS 117 patients were enrolled with 70% being female, an average age of 39.1 ± 13.3, and an average body mass index was 26.5 ± 5.4. The average labral sizes based on intraoperative measurements were 6.85 mm in zone 2, 7.45 mm in zone 3, and 7.29 mm in zone 4. The average labral sizes based on MRA were 6.95 mm in zone 2, 7.24 mm in zone 3, and 6.71 mm in zone 4. There was a poor correlation between MRA and intraoperative measurements in zones 2 and 3 (zone 2: R = 0.171, p = 0.065; zone 3: R = 0.335, p = 0.00022) and no correlation in zone 4 (R = -0.22, p = 0.82). CONCLUSIONS This study demonstrates a poor correlation in labral measurements between magnetic resonance arthrogram imaging and intraoperative measurements, suggesting that this imaging modality may be insufficient in providing accurate measurements of labral size.