Background: Hamstring autograft size <8 mm has been shown to be a predictor for failure after anterior cruciate ligament (ACL) reconstruction. The ability to predict graft size preoperatively is helpful in counseling patients about the possible need for graft augmentation. Purpose: To determine whether preoperative ultrasound (US) measurements of hamstring tendons can predict intraoperative graft diameter during ACL reconstruction. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: Twenty patients undergoing unilateral isolated ACL reconstruction were prospectively enrolled in the study (10 males, 10 females; mean ± SD age, 22.8 ± 6.6 years; height, 175.1 ± 7.1 cm; weight, 81.4 ± 14.2 kg; body mass index, 26.5 ± 4.1 kg/m2). Hamstrings were assessed by US, and double-looped semitendinosus-gracilis hamstring size was independently calculated with a freehand selection method on a nonmagnified US image by 2 orthopaedic surgeons. Intraoperative autograft size was determined with a standard graft-sizing tool. Intra- and interrater reliability was measured with intraclass correlation coefficients (ICCs) and standard error of the measure (SEM). A receiver operating characteristic curve was calculated to assess the ability of the US measurement to predict intraoperative measurements. Results: The mean autograft diameter by US was 8.9 ± 0.98 mm, while the mean intraoperative hamstring graft size was 8.1 ± 0.89 mm. There was excellent intrarater (ICC2,1 = 0.95, SEM = 0.32 mm) and interrater (ICC2,1 = 0.88, SEM = 0.55 mm) reliability for US measurements. Receiver operating characteristic analysis showed that US did not consistently quantify graft size. Graft size did not significantly correlate with height, weight, or body mass index in our sample (P >.05). Conclusion: These results suggest that preoperative US imaging of the hamstring tendons is unreliable in predicting intraoperative graft diameter.