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The purpose of this paper was to determine whether acromial morphology influences anteroposterior shoulder stability. We hypothesized that a more horizontal and higher position of the acromion in the sagittal plane would be associated with posterior instability.In this retrospective study, patients with unidirectional posterior instability were age and sex-matched to a cohort of patients with unidirectional anterior instability. Both cohorts were compared with a control group of patients with no instability and no degenerative glenohumeral (rotator cuff and/or joint surface) or acromial changes. Measurements on radiographs included posterior acromial tilt, anterior and posterior acromial coverage (AAC and PAC), posterior acromial height (PAH), and the critical shoulder angle (CSA).The number of patients enrolled in each instability group was 41, based on a priori power analysis. The control group consisted of 53 shoulders. Of the measured anatomic factors, PAH showed the most significant association with posterior instability (odds ratio [OR] = 1.8; p < 0.001) in the logistic regression model. PAH was significantly greater in the posterior instability group compared with the anterior instability group (30.9 versus 19.5 mm; p < 0.001). With a cutoff value of PAH of 23 mm, the OR for posterior instability was 39. Shoulders with posterior instability were also significantly different from normal shoulders with regard to PAH (p < 0.001), AAC (p < 0.001), and PAC (p < 0.001) whereas, in the shoulders with anterior instability, all of these values except the AAC (p = 0.011) did not differ from those of normal shoulders.Specific acromial morphology is significantly associated with the direction of glenohumeral instability. In shoulders with posterior instability, the acromion is situated higher and is oriented more horizontally in the sagittal plane than in normal shoulders and those with anterior instability; this acromial position may provide less osseous restraint against posterior humeral head translation. A steep “Swiss chalet roof-type” acromion virtually excluded recurrent posterior instability in an albeit relatively small cohort of patients. Additional investigation is needed to determine the relevance of these findings for future treatment.Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.