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Abstract Background Interstitial lung disease (ILD) is a rare adverse event in patients receiving adjuvant or neoadjuvant chemotherapy (NAC) for breast cancer. Few studies have reported the frequency of ILD in detail, and only small numbers of cases have been described in the literature. Given these previous findings concerning ILD, we retrospectively examined the clinicopathological characteristics of five cases of ILD who had received epirubicin and cyclophosphamide (EC) and compared their findings with non-ILD cases. Methods The present single-center retrospective study included breast cancer patients who underwent adjuvant chemotherapy or NAC at our hospital between January 2014 and January 2021. Results Thirty-nine patients who had received EC for operable breast cancer were enrolled in this study. ILD developed 5 out of 39 patients (12.8%). The incidence of ILD in patients with non-dose-dense (dd) or dd chemotherapy was statistically significantly different (p = 0.0149). ILD occurred in three patients during dd EC treatment and two during weekly paclitaxel (wPTX) after dd EC. ILD was detected in one patient with high Krebs von den Lungen-6 (KL-6) levels, in two patients with continuous pyrexia, and in two patients from computed tomography imaging, which was taken to estimate the efficacy of chemotherapy, in two patients. Three of the 5 ILD patients underwent bronchoalveolar lavage, and 2 of these patients were diagnosed with Pneumocystis jirovecii pneumonia (PCP). There were no cases of serious ILD that required steroid pulse therapy. Conclusions Dd chemotherapy may be associated with an increased ILD frequency, which may reflect developing PCP. Careful monitoring and a timely diagnosis are useful for detecting early-stage ILD.