Collecting duct carcinoma is an unusual variant of renal cell carcinoma, whose appearance and behavior are not well established. We identified six cases of collecting duct carcinoma in our files. The clinical, pathologic, and immunohistochemical characteristics of these tumors are reported. The most common symptom was gross hematuria (four cases). Two patients had cervical adenopathy due to metastatic tumor. Four rapidly developed systemic metastases and died within 4 to 24 months. The primary renal tumors were located predominantly in the renal medulla and pelvis and had a partially cystic white-gray appearance. Histologic examination showed prominent tubulopapillary structures, nests of clear cells, and infiltrating tubules in a dense desmoplastic stroma. Atypical hyperplastic changes were found in some of the adjacent collecting ducts. Mucicarminophilic material was present in glandular elements in all six cases. Immunohistochemical studies revealed positivity with antibodies to epithelial membrane antigen, keratins, peanut agglutinin, vimentin, Leu M1 and lysozyme. The location of this tumor in the medulla, its distinctive histologic appearance, mucin positivity, expression of high molecular weight cytokeratins, and peanut agglutinin suggest that this is a distinct clinicopathologic entity which has an aggressive clinical course.