Objective:To evaluate the expression of the genes COL1A1, COL1A2, COL3A1 and COL5A1 in the glenohumeral capsule of patients with traumatic anterior instability of the shoulder.Methods:Samples from the glenohumeral capsule of 18 patients with traumatic anterior instability of the shoulder were evaluated. Male patients with a positive grip test and a Bankart lesion seen on magnetic resonance imaging were included. All the patients had suffered more than one episode of shoulder dislocation. Samples were collected from the injured glenohumeral capsule (anteroinferior region) and from the macroscopically unaffected region (anterosuperior region) of each patient. The expression of collagen genes was evaluated using the polymerase chain reaction after reverse transcription with quantitative analysis (qRT-PCR).Results:The expression of COL1A1, COL1A2 and COL3A1 did not differ between the two regions of the shoulder capsule. However, it was observed that the expression of COL5A1 was significantly lower in the anteroinferior region than in the anterosuperior region (median ± interquartile range: 0.057 ±0.052 vs. 0.155 ±0.398; p = 0.028) of the glenohumeral capsule.Conclusion:The affected region of the glenohumeral capsule in patients with shoulder instability presented reduced expression of COL5A1.