Background <p>Upper tract urothelial carcinoma (UTUC) requires a&#xa0;complex, multimodal, and guideline-based therapeutic approach. However, due to the rarity of the disease, low case numbers, and diagnostic challenges, implementation of the annually updated European Association of Urology (EAU) guidelines in daily clinical practice is often difficult. The aim of this study was to evaluate guideline-compliant care and the influence of guideline-deviating treatment decisions on oncological outcomes.</p> Materials and methods <p>In a&#xa0;retrospective analysis, 181 patients with UTUC who were treated at our clinic between 2011 and 2024 were examined. Diagnosis, therapy, and oncological outcome were evaluated based on the applicable EAU guidelines, and treatment courses were analyzed in relation to guideline adherence.</p> Results <p>In all, 13% of patients received nonguideline-compliant diagnosis or therapy. The main causes were lack of bladder cuff resection (52%), lack of adjuvant chemotherapy (32%, 2020–2024), lack of interdisciplinary tumor board discussion (17%), and surgical undertreatment (5%). The lack of bladder cuff resection was associated with a&#xa0;significantly reduced overall survival (<i>p</i> = 0.047) and an increased recurrence rate (<i>p</i> = 0.038). In addition, there were repeated deviations in the comprehensive diagnostics for the correct classification of low-risk and high-risk patients.</p> Conclusion <p>Despite clear recommendations from the EAU, their implementation in everyday clinical practice remains challenging. However, guideline-based care is essential, as deviations are associated with significantly poorer oncological outcomes.</p>