High-grade papillary urothelial carcinoma with subepithelial connective tissue invasion (T1HG) is an aggressive disease at high risk of progression after transurethral resection/Bacillus Calmette-Guerin standardized therapy. The European Organization for Research and Treatment of Cancer has identified T1HG bladder carcinoma that is single and ≤3 cm in the largest dimension at first diagnosis as a category in which the prognosis cannot be further stratified based on conventional criteria. This category may benefit from biomarker analysis as a valuable tool to determine the patient's outcome. To further the issue of biomarkers in predicting aggressiveness in single T1HG bladder carcinoma ≤3 cm in greatest dimension at first diagnosis, we have conducted a validation study of the biomarker risk score set previously reported by our group. The study set included immunohistochemical detection of galectin-3, CD44, E-cadherin (E-CAD), CD138, p16, survivin, HYAL-1, and topoisomerase-IIα in 92 randomly selected specimens at participating institutions. Topoisomerase-IIα expression was identified as a predictor of disease-free survival. p16, survivin, and E-CAD expression predicted progression-free survival, but p16 and E-CAD also predicted overall survival. The current study validates a panel of immunohistochemical markers with the potential of being implemented in practice and supports the use of biomarkers in predicting aggressiveness in patients with first diagnosis of single T1HG bladder carcinoma ≤3 cm in greatest dimension and therefore in identifying patients who need closer surveillance or earlier aggressive treatment.