Abstract Aims The European Association of Urology guideline for upper tract urothelial carcinoma (UTUC) relies on two grading system: 1973 World Health Organization (WHO) and 2004/2016 WHO. No consensus has been made which classification should supersede the other and both are recommended in clinical practice. We hypothesized that one may be superior to the other. Methods Newly diagnosed non-metastatic UTUC patients treated with radical nephroureterectomy were abstracted from the Surveillance, Epidemiology, and End Results database (2010–2016). Kaplan–Meier plots and multivariable Cox regression models (CRMs) tested cancer-specific mortality (CSM), according to 1973 WHO (G₁ vs. G₂ vs. G₃) or to 2004/2016 WHO (low-grade vs. high-grade) grading systems. Haegerty’s C-index quantified accuracy. Results Of 4271 patients, according to 1973 WHO grading system, 134 (3.1%) were G₁, 436 (10.2%) were G₂ and 3701 (86.7%) were G₃; while according to 2004/2016 WHO grading system, 508 (11.9%) were low grade vs 3763 (88.1%) high grade. In multivariable CRMs, high grade predicted higher CSM (Hazard ratio: 1.70, p < 0.001). Conversely, neither G₂ (p = 0.8) nor G₃ (p = 0.1) were independent predictors of worse survival. The multivariable models without consideration of either grading system were 74% accurate in predicting 5-year CSM. Accuracy increased to 76% after either addition of the 1973 WHO or 2004/2016 WHO grade. Conclusions From a statistical standpoint, either 1973 WHO or 2004/2016 WHO grading system improves the accuracy of CSM prediction to the same extent. In consequence, other considerations such as intra- and interobserver variability may represent additional metrics to consider in deciding which grading system is better.