Background: This study assessed whether a standard 21-gauge needle (length, 38.1 mm [1.5 inches]) is more likely to penetrate the glenohumeral joint through an anterior or a posterior approach. Methods: Seventy-nine patients underwent an arthroscopic procedure on the glenohumeral joint. The depth from the skin to the joint capsule was compared between the posterior approach (10 mm medial and inferior to the posterolateral tip of the acromion) and the anterior approach (direct visualization through the rotator interval). Each approach was measured twice and the mean used. The data were analyzed using a 2-sided paired t test. Results: The anterior approach was shorter than the posterior approach in all patients (P < .001). This was less than the length of a standard needle in 98.7% of patients. The mean skin-to-joint capsule depth was 43.5 mm (range, 24-58 mm) with the posterior approach and 27.1 mm (range, 12.5-40 mm) with the anterior approach. On average, the posterior approach was 16.3 mm deeper (range, 0.5-31.5 mm) than the anterior approach. Conclusions: Injections through the anterior approach are more likely to penetrate the glenohumeral joint than through the posterior approach if a standard needle is used.