Introduction: Occult primary malignancy usually manifested itself as metastases or secondary–paraneoplastic phenomena. Although occult breast cancer usually presented as axillary lymphadenopathy, it could also have other rare manifestations, such as extensive osteolytic lesions which is the typical manifestation of Multiple Myeloma (MM). The similarity in radiological findings made differential diagnosis extremely difficult, especially when primary breast lesion was undetectable by ordinary diagnostic tools. Here we present a rare occult breast cancer case detected only by positron emission tomography (PET) with extensive osteolytic bone metastases that mimicked MM. Case description: A 48-year-old female patient presented with heart burn, dyspenia and lower back pain. Skeletal survey revealed extensive osteolytic lesions including skull, spine and pelvis (Figure 1A-C), which were highly suspicious for MM. However, there were no Ben-Jones protein in urine and blood sample with mild proliferative bone marrow and normal plasma cell phenotype. Hence, after ruling out MM, PET scan was arranged to screen potential malignancies. It revealed a high uptake lesion in left breast with SUV 2.7 (Figure 1D) and multiple metastases. Although the breast lesion had a high SUV on PET, it was undetectable by mam- mography and ultrasound. Conclusions: Extensive osteolytic lesions could be the first symptom of occult breast cancer. PET could be a useful tool for occult malignancies to identify primary lesion. Due to the relative low sensitivity of PET in detection of primary breast lesion, clinicians should be aware of occult breast cancer when PET revealed no implications for primary sites. Tissue biopsy of metastatic diseases could be another option to confirm the diagnosis. Take-home message: 1. Extensive osteolytic lesions could be the first symptom of occult breast cancer. 2. PET could be a useful tool to identify primary lesion of occult malignancies. 3. PET has a relative low sensitivity of primary breast lesion, negative result could not rule out occult breast cancer. 4. Tissue biopsy of metastatic diseases could be another option to confirm the diagnosis.