There is no consensus on the pathogenesis of rotator cuff tears. Fibrous cartilage has been hypothesized to develop in some tendons as a result of shear or compressive forces, resulting in a tissue less capable of resisting normal tensile load and more prone to tearing. To test the hypothesis that metaplastic fibrocartilage in the rotator cuff could be involved in the pathogenic mechanism of its tear, samples from 34 acute and chronic torn rotator cuffs were subjected to histologic and immunohistologic study for the presence and type of cartilage (hyaline, fibrous, or elastic) in the area of the lesion and surrounding tissues. Detection of type I and II collagen, S-100 protein, and chondroitin sulfate allowed areas of fibrous cartilage to be seen in all samples, suggesting that the characteristic of rotator cuff tendons to work both in tension and in compression may stimulate fibrocartilaginous metaplasia and lead to a complete tear.