Simple SummaryUpper tract urothelial carcinomas (UTUCs) occur in about 5–10% of all urothelial carcinomas and are frequently discovered in high-stage disease. We aimed to evaluate human epidermal growth factor receptor 2 (ERBB2) protein expression andERBB2amplification in UTUCs according to the American Society of Clinical Oncology/College of American Pathologists guidelines for gastric carcinoma, revealing an obvious higher rate of ERBB2 overexpression (41.8%) in contrast to the relatively low frequency ofERBB2amplification (10.5%) in UTUCs. Correlation and survival analyses show that ERBB2 overexpression and amplification were linked with high-grade and high-stage tumors and with tumor progression. The results suggest that ERBB2 is a biomarker for progression in UTUCs. As previously shown,ERBB2amplification is infrequent. However, the small number of patients diagnosed withERBB2-amplified UTUC might benefit from ERBB2-targeted therapy. For the reliable detection of rareERBB2-amplified UTUC, simultaneous immunohistochemical and cytogenetic ERBB2 analysis is recommended.AbstractUpper tract urothelial carcinomas (UTUCs) occur in about 5–10% of all urothelial carcinomas and are frequently discovered in high-stage disease. We aimed to evaluate human epidermal growth factor receptor 2 (ERBB2) protein expression immunohistochemically andERBB2amplification in UTUCs by fluorescence in situ hybridization, applying a tissue microarray technique. ERBB2 overexpression andERBB2amplification were defined according to the recommendations of the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) for breast cancer and gastric carcinoma (GC), revealing scores of 2+ and 3+ in 10.2% and 41.8% of UTUCs, respectively. The performance parameters showed obviously higher sensitivity of ERBB2 immunoscoring according to the ASCO/CAP criteria for GC.ERBB2amplification was detected in 10.5% of UTUCs. ERBB2 overexpression was more likely to be found in high-grade tumors and was associated with tumor progression. Univariable Cox regression analysis revealed a significantly lower progression-free survival (PFS) in cases with ERBB2 immunoscores of 2+ or 3+ according to the ASCO/CAP guidelines for GC. UTUCs withERBB2amplification showed a significantly shorter PFS in the multivariable Cox regression analysis. Irrespective of theirERBB2status, patients with UTUC treated with platin showed a significantly lower PFS than UTUC patients who had not received any platin-based therapy. In addition, UTUC patients with a normalERBB2gene status who had not received platin-based therapy showed significantly longer overall survival. The results suggest that ERBB2 is a biomarker for progression in UTUCs and may define a distinct subgroup of UTUCs. As previously shown,ERBB2amplification is infrequent. However, the small number of patients diagnosed withERBB2-amplified UTUC might benefit from ERBB2-targeted cancer therapy. In clinical–pathological routine diagnostics, the determination ofERBB2amplification is an established method in some defined entities and also successful in small samples. Still, the simultaneous use of ERBB2 immunohistochemistry andERBB2in situ hybridization would be important in order to record the low rate of amplified UTUC cases as completely as possible.