Background/Aim: Pneumonitis is a serious complication after radiotherapy of breast cancer. This study aimed to identify its prevalence and potential risk factors. Patients and Methods: A total of 606 patients irradiated following breast-conserving surgery or mastectomy were retrospectively analyzed. In patients developing pneumonitis, radiation and clinical parameters were investigated to identify potential risk factors. Results: Eleven patients (1.8%) developed a pneumonitis grade ≥2. Mean doses to the ipsilateral lung were >7 Gy in 5 patients (45%). Of the other patients, 5 had a chronic inflammatory disease. Six patients (55%) had another malignancy (4 previous contralateral breast cancers, 1 previous ovarian and thyroid cancer, 1 synchronous carcinoma-in-situ (pTis) at the contralateral breast). Five patients (45%) received chemotherapy including taxanes and 4 patients (36%) received trastuzumab. Conclusion: The prevalence of pneumonitis was 1.8%. Potential risk factors included mean radiation dose to ipsilateral lung >7 Gy, systemic treatment with taxanes or trastuzumab, chronic inflammatory disease and history of another malignancy.