Background: Many cases of organizing pneumonia (OP) after radiotherapy for breast conservation surgery have been reported. However, there are few reports on OP after bilateral breast irradiation. Case Report: A 45-year-old woman received conservation surgery for bilateral breast cancers. Irradiation was started two months after surgery to prevent postoperative recurrence, and both breasts were tangentially irradiated simultaneously at 2 Gy per fraction for a total of 50 Gy. The patient visited this hospital complaining of fever and dry cough 2.5 months after irradiation. CT revealed infiltrative changes with air bronchograms that corresponded to the irradiated field on the ventral sides of both lungs, as well as infiltration and ground-glass opacities that did not correspond to the irradiated field, and appeared to be spreading into areas adjacent to the infiltration in the irradiation fields. Conclusions: Meticulous follow-up appears to be necessary for patients receiving simultaneous bilateral breast irradiation because there is a possibility that irradiation-induced lesions will temporarily spread extensively.