Uroplakins (UPs) Ia, Ib, II, and III are tissue‐specific and differentiation‐dependent transmem‐brane proteins of the urothelium. We assessed the usefulness of human UP Ia as a histological marker by examining its expression in urinary bladder transitional cell carcinoma (TCC). A polyclonal antibody against human UP Ia was raised using a synthesized polypeptide. We applied our antibody to various organ tissues, including urothelium, and observed no crossreactivity. Analysis by RT‐PCR of normal urothelium, TCC and other organ tissues indicated that the human UP Ia gene expression is highly specialized to urothelium, and is conserved in TCC. Using immunohistochemistry, we investigated the expression of UP Ia in TCC from patients who had undergone radical cystectomy and from autopsy cases. Positive staining (10% or more positive cancer cells) was noted in primary lesions from 61 of 63 (96.8%) cystectomy patients. Depending on pathological grade, high expression (50% or more positive cancer cells) was observed in 17 of 18 (94.4%) moderately‐ to well‐differentiated TCC and in 36 of 45 (80.0%) poorly differentiated TCC. With regard to tumor invasion, high expression was noted in 20 of 22 (90.9%) superficial and 33 of 41 (80.5%) muscle‐invasive TCC. Cause‐specific survival rates were 68.6% and 75.0% in high‐ and low‐expression patients, respectively (log‐rank test, P=0.855, mean follow‐up; 65.0 months). In metastases, positive reactions were observed in 13 of 18 (72.2%) lesions. UP Ia may represent a specific histological marker judging from the stable expression, although its value as a prognostic factor remains undetermined.