Two hundred twenty shoulders with a rotator cuff tear repaired by simple tendon-to-bone suture were analyzed to determine whether the severity of presurgical fatty degeneration had an influence on their anatomic and functional outcome. Fatty degeneration was evaluated for each muscle with the 5-stage grading system developed by Goutallier et al. A global fatty degeneration index (GFDI), the mean value of the 3 muscles, was calculated for each shoulder. Cuff integrity was evaluated by magnetic resonance imaging (116 cases) or computed arthrotomography scan (104 cases) at a mean 37 months' follow-up, and functional outcomes were evaluated with the Constant score. A recurrent tear was found in 79 cases (36%) and was more frequently encountered in posterosuperior tears. The likelihood of a recurrent tear was greater for tendons whose muscle showed fatty degeneration greater than grade 1. Fatty degeneration of the infraspinatus or subscapularis muscles had an influence on supraspinatus tendon outcome. A GFDI lower than 0.5 was necessary to yield less than 25% retears. The mean global Constant score was 75 at revision, significantly lower when a retear was present (70.5 versus 77.5). In the subgroup of watertight cuffs, it was lower when GFDI was higher. Fatty degeneration is an important prognostic factor in rotator cuff surgery.