To assess the clinicopathological features and prognostic significance of the presence of 5% or less high-grade component in otherwise low-grade noninvasive bladder urothelial carcinoma, referred to as mixed-grade (MG) urothelial carcinoma, we reviewed all archival cases with such diagnosis between 2005 and 2014. Clinicopathological and outcome parameters were compared to those in our previously reported low- and high-grade noninvasive bladder urothelial carcinoma cohorts (LGUC and HGUC, respectively). The study included 31 MG urothelial carcinomas. Mean patient age was 67.6 years, and mean follow-up was 39.7 months. Intravesical treatment was administered in 15 patients (48.4%). Recurrence occurred in 14 cases (45.2%): 10 as LGUC and 4 as HGUC; there was no stage progression. Mean time to progression was 9 months (5-17 months), and there was no death of disease. MG urothelial carcinoma stage progression and dead of disease rates were comparable to that of LGUC. MG urothelial carcinoma stage progression was significantly lower than that of HGUC, P =.002, using Pearson chi(2) test. MG urothelial carcinoma patients with no intravesical treatment had higher incidence rate of grade progression (25%) compared to LGUC patients (7.9%); however, the difference was not statistically significant. MG urothelial carcinoma had a prognosis closer to "pure" LGUC than "pure" HGUC. Untreated MG urothelial carcinoma may have a higher rate of grade progression than LGUC, although more data are needed before this issue can be defmitively addressed. Until such data are available, it is reasonable to keep MG urothelial carcinoma as a distinct grade category with potential management implications. (C) 2015 Elsevier Inc. All rights reserved.