Recently several low-grade renal cell tumors, distinct from those recognized by the 2004 World Health Organization classification of renal tumors, have been described. These tumors had similar clinicopathologic features, being low-stage tumors with cystic, tubuloacinar, and/or papillary architecture. The tumor cells were low grade with variable amounts of clear cytoplasm that was positive for cytokeratin 7 (CK7), but negative for CD10. Genetic changes characteristic of clear cell or papillary renal cell carcinoma were not seen in these tumors. We investigated the morphologic, immunohistochemical, and genetic features of 36 additional tumors. Immunohistochemistry was carried out for CK7, carbonic anhydrase 9, α-methylacyl-CoA racemase, CD10, TFE-3, and desmin. Interphase fluorescence in situ hybridization was carried out with centromeric probes for chromosomes 3, 7, 17, and a subtelomeric probe for 3p25. Sequencing of von Hippel-Lindau gene and analysis of the methylation status of the promoter region was also carried out in 2 tumors. Thirty-six tumors from 33 patients (mean age: 60.4, range: 26 to 88; 17 men and 16 women) were studied. Three patients had bilateral tumors and 1 patient had von Hippel-Lindau disease. Follow-up was available in 60% (20/33) of the patients for a mean of 27.4 (range 1 to 85) months. No patient had evidence of the disease after surgery except for the patient with von Hippel-Lindau disease, who was alive with stable disease in the contralateral kidney. All 36 tumors were small (mean size 2.4cm; range 0.9 to 4.5cm) and low stage (pT1). The majority was cystic and had prominent fibrous capsule and stroma. The tumors were composed of variable amount of cysts, papillae, tubules, acini, and solid nests. The most characteristic histologic features were branching tubules and acini and anastomosing clear cell ribbons with low-grade nuclei. All tumors were strongly positive for CK7 and variably positive for CA9, but largely negative for CD10, and negative for α-methylacyl-CoA racemase and TFE-3. All but 1 tumor had no gains of chromosomes 7 and 17 and deletion of 3p. Only 1 tumor had low copy number gains of chromosomes 7 and 17. VHL gene mutation and promoter methylation were negative in 2 tumors analyzed. We show that these tumors, which we term as "clear cell tubulopapillary renal cell carcinoma," constitute a unique subtype in the spectrum of renal epithelial neoplasia based on their characteristic morphologic and immunohistochemical features. © 2010 by Lippincott Williams & Wilkins.