Breast cancer is the most common cancer and the second leading cause of cancer deaths among women in developed countries. Bone is a frequent site of metastatic disease with a stage-dependent incidence. Most women with breast cancer are at risk of osteoporosis due to their age or their breast cancer treatment. Scintigraphy enables imaging of the entire skeleton with high sensitivity but limited specificity. The false positive rate varies from 1.6% to as high as 22%, while the false negative rate varies from 0.96% to 13%. We observed a 70-year-old woman with a diagnosis of breast cancer and a false negative bone scan despite extensive bone metastases. She was under alendronate treatment for osteoporosis at the time. The false negative finding might be due to a transient phenomenon of alendronate, a bisphosphonate cleared from the plasma by uptake into bone and by renal excretion. 99mTc-MDP is eliminated via the same pathways, and therefore competition may occur between the two substances. Another possible explanation for the false negative bone scan could be that bone metastases, indicating hematogenous tumor spread, are detected earlier by CT scan or MRI than by bone scan. Breast cancer patients under bisphosphonate treatment for osteoporosis must be carefully evaluated for bone metastasis during radionuclide studies with 99mTc-MDP.