Small cell carcinoma is a malignant tumor composed of small cells with a diffuse growth pattern. It has immunohistochemical and ultrastructural features of both neuroendocrine and epithelial neoplasms. These tumors constitute 10% to 20% of malignant tumors in the lung, which is their most frequent site. They have been described in other extrapulmonary sites, where they are defined using the same criteria as used in the lung. These tumors are rarely found in the genitourinary tract. Fewer than 30 cases have been reported in the kidney. The differential diagnosis mainly includes other small round cell tumors and metastatic small cell lung carcinoma. We present and discuss a primary small cell carcinoma of the kidney (to our knowledge the 9th to be reported in the literature), which we studied with light microscopy, immunohistochemistry, electron microscopy, and, for the first time, flow cytometry.