Background In selected patients with a desire to maintain activity levels greater than those recommended after reverse total shoulder arthroplasty, hemiarthroplasty remains an option for treatment of cuff tear arthropathy (CTA). However, given the relatively small case series that have been reported to date, little is known regarding which patients will show functional improvement after this surgery.Questions/purposes We asked: What factors are associated with achieving the minimum clinically important difference in the simple shoulder test (SST) after hemiarthroplasty for cuff tear arthropathy? Patients and Methods Between 1991 and 2007, two surgeons at one academic center performed 48 shoulder hemiarthroplasties for CTA. No patients were known to have died before data collection, and of those not known to have died, 42 (88%) were available for followup at a mean of 48 months (range, 24-132 months). During that time, the general indications for this approach were glenohumeral arthritis with superior decentering of the humeral head. The majority of the patients with CTA were treated nonoperatively with patient-directed physical therapy and other modalities. A total of 42 patients (42 shoulders; 24 males and 18 females) with CTA were treated with hemiarthroplasty and followed for a mean of 48 months (range, 24-132 months). This is a retrospective study that made use of a longitudinally maintained database, which included physical examination of ROM, the SST, VAS, and standardized radiographs. At latest followup, 33 of 42 patients achieved a clinically important percentage of maximum possible improvement (%MPI) in SST score, One of the authors certifies that he (KB), or a member of his immediate family, has or may receive payments or benefits, during the study period, an amount of less than USD 10,000 from DepuySynthes (Warsaw, IN, USA). One of the authors certifies that he (CAR), has or may receive payments or benefits, during the study period, an amount of more than USD 1,000,001 from Depuy-Synthes (Warsaw, IN, USA). One of the authors certifies that he (MAW), has or may receive payments or benefits, during the study period, an amount of USD 100,001-1,000,000 from Depuy-Synthes (Warsaw, IN, USA) and less than USD 10,000 from Tornier (Lyon, France), Saunders (Philadelphia, PA, USA) and Arthrex (Naples, FL, USA). All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research 1 editors and board members are on file with the publication and can be viewed on request. Clinical Orthopaedics and Related Research 1 neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDAapproval status, of any drug or device prior to clinical use. Each author certifies that his or her institution approved the human protocol for this investigation, that all investigations were conducted in conformity with ethical principles of research, and that informed consent for participation in the study was obtained....