BACKGROUND Renal tumors are often diagnosed during routine radiological-imaging. A newly diagnosed renal tumor next to an existing cancer is challenging since a primary or a secondary renal neoplasm has to be considered in the differential diagnosis. METHODS A 64-year-old woman underwent radical mastectomy and axillary lymphadenectomy for cancer of the right breast. After surgery, the patient underwent chemotherapy and radiotherapy because of multiple metastases. Six years later, computed tomography (CT) obtained as follow-up examination revealed a solid mass in the left kidney. Because radiological differentiation between metastatic breast cancer and primary kidney tumor was impossible and fine needle biopsy of renal tumors should be avoided, a nephrectomy was performed. Histologic diagnosis was a metastatic breast cancer within a primary renal cell carcinoma. Whereas the primary tumor was receptor negative, the breast cancer metastasis was estrogen receptor positive. CONCLUSIONS In a renal mass of unknown nature in patients in good general condition and acceptable life-expectancy, surgical exploration with partial or radical nephrectomy is justified in spite of a synchronous metastatic tumor of different origin. This is the only way to obtain a definitive histologic diagnosis. A primary renal tumor can be treated curatively, preventing secondary complications, such as hematuria. In this case, the changed receptor state of the breast cancer metastasis also offered the patient the possibility of new palliative chemotherapy and hormonal manipulation.