AbstractPreviously, we have demonstrated that interleukin‐4 receptor α (IL‐4Rα) is overexpressed on a variety of human cancers and can serve as target for IL‐4 immunotoxin comprised of IL‐4 and a mutated Pseudomonas exotoxin. However, its expression and association with grade and clinical stage of bladder cancer has not been studied. IL‐4Rα expression was examined in human bladder cancer cell lines, mouse xenografts, and biopsy specimens at mRNA and protein levels by real‐time RT‐PCR and IHC/ISH techniques. We also examined the effect of IL‐4 on proliferation and invasion of bladder carcinoma cell lines. For tissue microarray (TMA) results, we analyzed the precision data using exact binomial proportion with exact two‐sided P‐values. We used Cochran–Armitage Statistics with exact two‐sided P‐values to examine the trend analysis of IL‐4Rα over grade or stage of the bladder cancer specimens. The influence of age and gender covariates was also analyzed using multiple logistic regression models. IL‐4Rα is overexpressed in five bladder cancer cell lines, while normal bladder and human umbilical vein cell lines (HUVEC) expressed at low levels. Two other chains of IL‐4 receptor complex, IL‐2RγC and IL‐13Rα1, were absent or weakly expressed. IL‐4 modestly inhibited the cell proliferation, but enhanced cell invasion of bladder cancer cell lines in a concentration‐dependent manner. Bladder cancer xenografts in immunodeficient mice also maintained IL‐4Rα overexpression in vivo. Analysis of tumor biopsy specimens in TMAs revealed significantly higher IL‐4Rα immunostaining (≥2+) in Grade 2 (85%) and Grade 3 (97%) compared to Grade 1 tumors (0%) (P ≤ 0.0001). Similarly, 9% stage I tumors were positive for IL‐4Rα (≥2+) compared to 84% stage II (P ≤ 0.0001) and 100% stages III–IV tumors (P ≤ 0.0001). IL‐13Rα1 was also expressed in tumor tissues but at low levels and it did not show any correlation with the grade and stage of disease. However, the IL‐2RγC was not expressed. Ten normal bladder specimens demonstrated ≤1+ staining for IL‐4Rα and IL‐13Rα1 and no staining for IL‐2RγC. These results demonstrate that IL‐4Rα is overexpressed in human bladder cancer, which correlates with advanced grade and stage of the disease. Thus, IL‐4Rα may be a bladder tumor‐associated protein and a prognostic biomarker.