MUC1 (epithelial membrane antigen) is a membrane-associated mucin known to interfere with both cell-cell and cell-matrix adhesions. Overexpression has been associated with poor prognosis in a variety of cancers. We investigated the expression of MUC1 (using two different antibodies, MA695 and E29) and E-cadherin in renal cell carcinomas (137 conventional, 23 chromophobe, 20 papillary, and eight unclassified tumors) with respect to diagnostic and prognostic significance using a tissue microarray technique. Immunoreactivity was correlated with histological subtype, pT-stage, and grade using the v 2 test or the Fisher's exact test, respectively. Impact on disease-free survival was analyzed using the Kaplan-Meier method and the log-rank test. Immunoreactivity of more than 10% of cancer cells with MA695, E 29, and E-cadherin antibodies was found in 112/133 (84%), 86/133 (65%), and 7/131 (5%) conventional, 20/22 (91%), 19/22 (86%), and 21/22 (95%) chromophobe, 13/20 (65%), 8/20 (40%), and 3/20 (15%) papillary as well as 5/8 (63%), 5/8 (63%), and 4/8 (50%) unclassified carcinomas, respectively. The two different MUC1 antibodies yielded comparable staining results. A diffuse cytoplasmic staining pattern for MUC1 was found exclusively in chromophobe carcinomas, whereas conventional and papillary subtypes showed predominantly membranous staining (Po0.0001). Regarding papillary carcinomas, MUC1 was predominantly associated with type 1 (P ¼ 0.0001), and E-cadherin with type 2 (P ¼ 0.049) tumors. The cellular staining pattern of MUC1 in conventional tumors was related to pT-stage (P ¼ 0.002) and tumor grade (P ¼ 0.001): Low-stage (pT1/pT2) and grade (G1/G2) tumors showed a predominantly apical membranous staining, high-stage (pT3a/pT3b) and grade (G3/G4) tumors a predominantly circumferential membranous staining (with or without additional diffuse cytoplasmic immunoreactivity), which, in the conventional subtype, was associated with poor prognosis (Po0.0001). In conclusion, MUC1 and E-cadherin are diagnostically and prognostically useful markers in renal tumor pathology, especially when cellular staining patterns are considered. Keywords: kidney; cancer; MUC1; EMA; E-cadherin; differential diagnosis; prognosisMucins are high-molecular-weight (4200 kDa) glycoproteins with oligosaccharides attached to an apomucin protein backbone (core peptide) by Oglycosidic linkage.