The purpose of this study was to evaluate fatty infiltration of supraspinatus quantitatively using fat fraction measurements and investigate the relationship between fatty infiltration, muscle atrophy and tendon tear severity. Quantitative assessment of fatty infiltration was performed using 3T MRI with a mDIXON-Quant technique in 85 patients. The degree of fatty infiltration and muscle atrophy signiﬁcantly increase with the extent of rotator cuff tear. The tendon tear severity correlated with fatty infiltration and muscle atrophy, and muscle atrophy correlated with fatty infiltration. mDIXON-Quant can be a reliable and accurate tool to quantify fatty degeneration of the rotator cuff muscles.