Chronic shoulder pain is a common complication in breast cancer patients after surgery. Chronic shoulder pain after breast cancer surgery was formerly considered as neuropathic pain, however the pathophysiology including structural damages has not been assessed comprehensively. We hypothesized that the structural change could be one of the cause of shoulder pain after breast cancer surgery and evaluated various ultrasonography findings of the shoulder in breast cancer patients with chronic shoulder pain. Patients who were suffering from chronic shoulder pain on unilateral side for at least 3 months after breast cancer surgery were enrolled from a single tertiary hospital. Demographic and clinical data were collected at the baseline. Articular and adjacent structures of both shoulders (painful and contralateral side) were evaluated by ultrasonography. The ultrasonography findings were compared between painful and contralateral sides. Logistic regression analysis was performed to determine the factors associated with abnormal ultrasonography findings. Fifty-two female patients (average age of 55) were enrolled. Significantly more abnormal ultrasonography findings were observed in the painful side than in the contralateral side [39 (75.0%) vs 11 (21.2%), P < 0.001]. The coracohumeral ligament was significantly thicker in the painful side than in the contralateral side (2.48 ± 0.69 vs 1.54 ± 1.25 mm, P < 0.001); adhesive capsulitis was also more frequent in the painful side [14 (26.9%) vs 0, P < 0.001]. Furthermore, patients with a history of breast cancer surgery on the ipsilateral side were associated with abnormal ultrasonography findings and adhesive capsulitis. This study is the first to evaluate ultrasonography in patients with chronic shoulder pain after breast cancer surgery. The results showed that ultrasonography could reveal several structural problems in these patients.