Aims To compare the 1973 WHO and the 2004/2016 WHO grading systems in patients with urothelial carcinoma of urinary bladder (UCUB), since no consensus has been made which classification should supersede the other and since both are recommended in clinical practice.

Methods Newly diagnosed patients with Ta UCUB treated with transurethral resection of bladder tumour 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) and to 2004/2016 WHO (low-grade vs high-grade) grading systems.

Results Of 35 986 patients, according to 1973 WHO grading system, 8165 (22.7%) were G1, 17 136 (47.6%) were G2 and 10 685 (29.7%) were G3. According to 2004/2016 WHO grading system, 24 961 (69.4%) were low-grade versus 11 025 (30.6%) high-grade. In multivariable CRMs, G3 (HR: 2.05, p<0.001), relative to G1, and high-grade(HR: 2.13, p<0.001), relative to low-grade, predicted higher CSM. Conversely, G2 (p=0.8) was not an independent predictor. The multivariable models without consideration of either grading system were 74% accurate in predicting 5-year CSM. After addition of 1973 WHO or 2004/2016 WHO grade, the accuracy increased to 76% and 77%, respectively.

Conclusions From a statistical standpoint, it appears that the 2004/2016 WHO grading system holds a small, although measurable advantage over the 1973 WHO grading system. Other considerations, such as intraobserver and interobserver variability may represent an additional matric to consider in deciding which grading system is better.