<b><i>Objective:</i></b> To determine the impact of prognostic factors of a series of high-grade Ta non-muscle-invasive bladder cancers (NMIBCs) according to the new International Society of Urological Pathology (ISUP) 1998/WHO 2004 grading system (previously classified as either TaG2 or TaG3). <b><i>Methods:</i></b> One hundred and thirty-one high-grade Ta (105 G2 and 26 G3) cases were identified after independent review by two pathologists. Univariable and multivariable Cox regression models addressed recurrence and progression-free survival. Progression was defined as appearance of any T ≥1 recurrence after complete TUR (type 1) or occurrence of T ≥2 (type 2). <b><i>Results:</i></b> Ten-year recurrence, type-1 and type-2 progression-free survival were 60, 75 and 95%, respectively. The previous grading system (G3 vs. G2) significantly predicted type 1 progression in the univariate model only. In the multivariate model, Ki67 was the only independent predictor of progression according to both definitions (HR = 5.25, p = 0.002 and HR = 6.16, p = 0.03, respectively). <b><i>Conclusions:</i></b> High-grade Ta NMIBC as defined by the WHO 2004 grading system cannot be equated with high-risk NMIBC. The risk of progression to muscle-invasive disease (type 2) is low, more in keeping with an intermediate-risk category of NMIBC. The previous WHO 1973 subcategorization into G2 and G3 is of little help in the prediction of outcome. Ki67 is a strong independent predictor of progression worthy of consideration for a clinical setting.