Forty-one patients scheduled for shoulder surgery underwent computed tomography (CT) and magnetic resonance imaging (MRI) examination of their affected shoulder to verify whether fatty degeneration of the rotator cuff muscles could reproducibly be assessed by CT or by MRI and whether the grading with the 2 methods was comparable. In addition, rotator cuff muscle cross-sectional areas were measured on parasagittal MRI scans to establish a possible correlation between rotator cuff muscle atrophy and fatty degeneration. Interobserver reproducibility for grading fatty degeneration was good to excellent for CT and for MRI. The correlation between MRI and CT was fair to moderate and remained unsatisfactory, even if the classification system was simplified with only a 3- rather than a 5-grade scale as originally proposed. The degree of fatty degeneration was significantly related to the amount of atrophy of the respective muscles.