Early detection and prompt treatment of breast cancer are the main tools to decrease mortality rates, Several diagnostic techniques such as mammography, magnetic resonance imaging, and ultrasound are used, but none of these are conclusive. The authors describe a 56-year-old woman who was suspected of having breast cancer on mammography. Tc-99m MIBI scintimammography showed a small mass with elevated radiotracer uptake located near the pectoral muscle of the patient's left breast. The possible tumor was confirmed with fine-needle aspiration biopsy, After the diagnosis was established, the patient did not allow any treatment. Eighteen months later, a second scintimammography was performed, showing a large mammary tumor with a high radiotracer uptake, a new small lesion located in the left upper quadrant, and axillary lymph node involvement. Chemotherapy treatment was instituted and after treatment, Tc-99m MIBI scintimammography was normal. This case report provides further evidence for Tc-99m MIBI scintimammography for the detection of breast cancer as well as to evaluate the response to chemotherapy.