Radiation-induced organizing pneumonia (OP) reportedly occurs in ~2% of patients who receive whole-breast radiotherapy (WBRT). Though there are several reported risk factors, they remain unclear and controversial. We analyzed the incidence of and risk factors for OP after WBRT at our institution. We analyzed 665 breast cancer patients (with WBRT of 679 breasts) who underwent WBRT from October 2007 to September 2012 at our institution and were followed up for more than 1 year after completion of WBRT. Factors included in the analysis were age, the side affected, central lung distance (CLD), radiation dose, concurrent endocrine therapy, and chemotherapy. The median age was 56 years (range, 23-89 years). The sides affected were left, right and bilateral in 342, 309 and 14 patients, respectively. The median CLD was 1.1 cm (range, 0-3.0 cm). Concurrent endocrine therapy was performed in 236 patients, and chemotherapy was given in 215 patients; of these, 4 received concurrent chemotherapy. OP developed in nine patients (1.4%). The median time taken to develop OP after the completion of WBRT was 4 months (range, 2-12 months). All nine patients were treated with steroids, and symptoms promptly improved, except in two patients who relapsed. Statistical analysis revealed that only CLD (≥1.5 cm) was significantly associated with the development of OP (P = 0.004). In conclusion, the incidence of OP after WBRT was 1.4%, and CLD was a significant risk factor. In these patients, OP was controlled with steroid administration.