This study aimed to evaluate atrophy of the latissimus dorsi muscle (LDM) via computed tomography (CT) in patients who underwent mastectomy and axillary lymph node dissection (ALND). Secondary objectives were to investigate the relationship between the number of lymph nodes and muscle atrophy and to evaluate a possible correlation of muscle atrophy with the time interval between surgery and CT. The data of 125 patients who underwent unilateral modified radical mastectomy and ALND were reviewed retrospectively. The thickness and density of LDM were assessed using thorax computed tomography. The non-surgical side served as the control for each patient. The number of lymph nodes removed at ALND and the time from surgery to CT scan were also reported and evaluated for correlation with muscle atrophy. The mean thickness and density of the LDM were statistically significantly lower on the surgical side compared to the non-surgical side (p< 0.0001). The number of lymph nodes removed at ALND was negatively correlated with the thickness and density of LDM. The time interval between surgery and computed tomography was not correlated to either the thickness or density of LDM (p>0.05). The thickness and density of the LDM were seen to be decreased due to surgery although preservation of the nervus thoracodorsalis was defined in the surgery report. Intraoperative injury of the nerve during surgery may present more commonly than expected and affect muscle atrophy. In addition, the immobility of patients with breast cancer due to pain and swelling may also cause muscle atrophy.