Immunofluorescent method by flow cytometry was used to quantify the expression of the tumor-associated protein βIII-tubulin (TUBB3) in the tissue of urothelial bladder cancer and visually normal mucosa (56 samples in total). The expression of the marker was detected in 100% of cases, and heterogeneity of the TUBB3 expression level both in tumor tissue and in "normal" mucosa was revealed. The level of TUBB3 in the "normal" mucosa did not depend on the distance from the tumor (1 cm or more than 3 cm) and, on average, it was lower than in the tumor tissue (21.8 ± 10.8% and 24.9 ± 13.2% vs 35.2 ± 12.4%; p = 0.04 and 0.005, respectively). An increase of the TUBB3 expression in the tumor and in the "normal" mucosa was revealed in muscle invasive bladder cancer compared to non-muscle invasive bladder cancer. Therefore, in urothelial bladder cancer, the tumor-associated protein TUBB3 is a molecular marker of bladder mucosa involvement in the malignancy process and predicts the risk of tumor muscle invasion, which may influence indications for early cystectomy.