Breast cancer is a common malignancy and a common cause of cancer-related mortality in women. Pre-treatment workup of breast cancer does not routinely include positron emission tomography scans. We aimed to review cases of women with breast cancer and a synchronous second primary malignancy. We present three cases of women with non-metastatic cancer in whom a synchronous second primary malignancy was found. Synchronous, second primary malignancies which were identified included rectal cancer, gastrointestinal stromal tumor, and non-small cell lung cancer. All second primary malignancies were identified by a PET-CT scan. In conclusion, PET-CT may be used for detection of secondary primary malignancies in select breast cancer patients.