The purpose of this study is to determine the incidence of primary breast cancer (PBC) detected on (18) F-fluorodeoxyglucose (FDG) positron emission tomography (PET)-computed tomography (CT) in patients with a known diagnosis of non-mammary malignancies. A database search was performed to identify patients with non-mammary malignancies who had undergone staging with FDG PET-CT at a single institution between September 2005 and September 2011 and with the word "breast" reported in the PET-CT dictation. Additional breast imaging studies, clinical data, and the final histopathology of the breast lesions were correlated with the PET-CT images. Of 1951 patients who underwent PET/CT, 440 incidental breast lesions were identified in 438 patients. Of these 440 lesions, 195 (45 %) were benign, 160 (37 %) malignant, and 85 (19 %) missing data. A total of 25 PBCs were diagnosed; with a median size of 1.8 cm (range 0.8-10.7 cm); and a median SUVmax of 4.4 (range 1.7-17.6). There were 19 invasive ductal cancers, 1 invasive lobular cancer, 2 papillary cancers, 1 tubular cancer, 1 sarcomatoid cancer, and 1 ductal carcinoma in situ. Eight patients had regional nodal disease. Mammography revealed the PBC in 19 of 23 tumors (83 %), sonography in 22 of 23 (96 %). Six percent (25 of 440) of incidental breast lesions identified on FDG PET-CT represent PBCs; more than half were at an early stage and potentially curable.