Background: Although many investigators have evaluated the accuracy of ultrasonography and magnetic resonance imaging for the detection of full and partial-thickness rotator cuff tears, few have directly compared the two tests. The purpose of our study was to compare the accuracy of the two tests for detection and measurement of the size of rotator cuff tears, with arthroscopic findings used as the standard. Methods: One hundred and twenty-four consecutive patients with shoulder pain were prospectively studied with ultrasonography and magnetic resonance imaging. Seventy-one had subsequent arthroscopy, and they formed the study group. The arthroscopic diagnosis was a full-thickness tear in forty-six patients, a partial-thickness tear in nineteen, and no tear in six. The presence or absence of a full or partial-thickness tear and the tear size as demonstrated by each imaging test and at the time of arthroscopy were recorded. The findings of the imaging tests and arthroscopy were then compared for each parameter. Results: Ultrasonography correctly identified forty-five of the forty-six full-thickness tears and magnetic resonance imaging, all forty-six. Ultrasonography correctly identified thirteen of the nineteen partial-thickness rotator cuff tears and magnetic resonance imaging, twelve of the nineteen. The overall accuracy for both imaging tests was 87%. Ultrasonography correctly predicted the degree of retraction of 73% of the full-thickness tears and the length of 85% of the partial-thickness tears, and magnetic resonance imaging correctly predicted the retraction and length of 63% and 75%, respectively. Ultrasonography correctly predicted the width of 87% of the full-thickness tears and 54% of the partial-thickness tears, and magnetic resonance imaging correctly predicted the width of 80% and 75%, respectively. No significant differences between ultrasonography and magnetic resonance imaging were demonstrated (p > 0.05). Conclusions: Ultrasonography and magnetic resonance imaging had comparable accuracy for identifying and measuring the size of full-thickness and partial-thickness rotator cuff tears. When an investigator has comparable experience with both imaging tests, the decision regarding which test to perform for rotator cuff assessment does not need to be based on accuracy concerns. The choice can be based on other factors, such as the importance of ancillary clinical information (regarding lesions of the glenoid labrum, joint capsule, or surrounding muscle or bone), the presence of an implanted device, patient tolerance, and cost. Level of Evidence: Diagnostic study, Level I-1 (testing of previously developed diagnostic criteria in series of consecutive patients [with universally applied reference "gold" standard]).