Entrapment of the suprascapular nerve is frequently overlooked in the differential diagnosis of shoulder pain. The diagnosis is typically not considered until patients develop severe weakness secondary to atrophy of the spinatus (spinous) musculature that the nerve supplies. Twenty-seven masses were identified adjacent to the suprascapular nerve on magnetic resonance (MR) images of the shoulder; there were 21 ganglion cysts, two synovial sarcomas, one Ewing sarcoma, one chondrosarcoma, one metastatic renal cell carcinoma, and one hematoma associated with a fracture. Atrophy of both the supraspinatus and infraspinatus muscles was seen in association with anteriorly located masses and proximal entrapment of the nerve in 11 cases (40%); isolated atrophy of the infraspinatus muscle was seen in association with posteriorly located masses and distal entrapment of the nerve in nine cases (33%). MR imaging may facilitate the diagnosis of suprascapular nerve entrapment in patients with shoulder pain of unclear origin when perineural masses and atrophy of the spinatus musculature are present.