BACKGROUND: Chest CT after a mastectomy in breast cancer patients is usually performed to detect recurrence or metastasis. It is essential for surgeons an radiologists to know the appearance of the chest wall in a postmastectomy state. Especially, muscle atrophies of the chest wall, such as that of the pectoralis major muscle, indicate whether the modified radical mastectomy (MRM) was properly performed or not. MATERIALS AND METHODS: We performed a retrospective analysis of chest wall changes in mastectomy patients with follow-up chest CT finding to evaluate the results of MRMs. The medical records and CT findings of chest wall atrophy for a total of 38 patients who were treated at Kangnam St. Mary's Hospital, the Catholic University of Korea, were reviewed. The mean age was 58.3 years, and the interval between operation and follow-up CT was 6.6 years. The operations performed were a radical mastectomy (RM) in 3 patients and a MRM in 35 patients: Petey in 28 patients, Auchincloss in 6 patients, and Scanlon in 1 patient. RESULTS: In the RM, muscle atrophies were noted at the m, subscapularis in 3 patients (100%), the m. serratus anterior in 2 patients (66.7%), and the m. lattisimus dorsi in 1 patient (33.3%). In MRM, the Patey and Auchincloss operations showed m. pectoralis major atrophies in 22 patients (78.8%) and 5 patients (83.3%) respectively. Additionally m. pectoralis minor atrophy was noted in all 6 patients (100%) who underwent the Auchincloss operation. However. there was no muscle atrophy in patients who received Scanlon operation. CONCLUSIONS: These results suggest that lateral pectoral nerve injury is the main cause of m. pectoralis major atrophy in a MRM. The Scanlon operation which preserves the serve by dividing the pectoralis minor m. at its origin is a suitable operation for chest-wall muscle preservation. We hope that by the future study, we can confirm the best way of preventing chest-wall muscle atrophy in MRMs.