A 54-year-old metastatic breast cancer patient who had undergone chemotherapy and radiotherapy was transferred to our hospital for further treatment. We utilized (18)F-fluoroestradiol (FES) to assess the estrogen receptor (ER) status of metastatic lesions. Interestingly, high accumulation of (18)F-FES in pneumonia caused by radiation was detected. Hence, this draws oncologists' attention to the possible false-positive result of (18)F-FES, which may lead to inappropriate endocrine therapy.