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Introduction The question of our study was to evaluate the incidence of coexisting outlet impingement among patients with calcifying tendinitis and a failure of the conservative treatment using intraoperative in addition to radiological criteria. This question is of clinical relevance as there is still a discussion about whether an additional arthroscopic subacromial decompression (ASD) should be performed when arthroscopic removal of the calcific deposits is needed. Materials and methods From February 2017 to 02/2018, we prospectively enrolled 50 patients who needed shoulder arthroscopy for calcifying tendinitis. We evaluated the x-rays for outlet impingement, measuring the lateral acromial angle (LAA), acromion slope, acromion index (AI) and Bigliani-type of the acromion. During shoulder arthroscopy, we evaluated the acromial undersurface for a bony impingement using a standardized view. If impingement was present, we performed an additional ASD. Before surgery, three months later and at final follow-up 20 months after surgery, we determined the relative Constant Score (rCS). Results 92.5% of the patients had an intraoperatively confirmed outlet impingement, whereas 82.5% of the patients had a radiological outlet impingement. The sensitivity of the radiographs was 83.8%, the specificity 33.3% when combining all parameters. The mean rCS improved significantly from 45% to 100% at final follow-up. Conclusion Our results show that coexisting outlet impingement is very common in patients with calcifying tendinitis and failure of the conservative treatment. As the radiological diagnosis is uncertain, an intraoperative assessment for impingement is rational and justifies an additional ASD.