BACKGROUND: The Goutallier classification of rotator cuff fatty infiltration is an accepted standard, yet no recommendations exist for which computed tomography plane is best to identify fatty infiltration of the supraspinatus. Our purpose was to determine the most reliable plane to evaluate supraspinatus fatty infiltration, assess reliability of the tangent sign, and to correlate fatty infiltration and muscle atrophy. METHODS: Fatty infiltration in 87 computed tomography scans was reviewed by 3 shoulder surgeons using the 5-tiered Goutallier classification and a separate 3-grade scale. The supraspinatus muscle was evaluated in the axial, coronal, and sagittal plane. The tangent sign was used to assess muscle atrophy. RESULTS: The axial plane produced the highest agreement for both the 5-tiered and 3-tiered systems. An objective radiographic marker was described to reliably determine grade 3 fatty infiltration. The tangent sign produced excellent agreement for the presence of muscle atrophy. A significant relationship between the tangent sign and fatty infiltration was discovered (P < .0001); grades 3 and 4 fatty infiltration correlated statistically with supraspinatus atrophy. CONCLUSION: The tangent sign is acceptable for determining the presence of muscle atrophy and clinical decision making. A positive tangent sign is an indicator of advanced fatty infiltration. The axial computed tomography plane should be used when evaluating fatty infiltration. LEVEL OF EVIDENCE: Level 3; Diagnostic study.