Background: Grading upper tract urothelial carcinomas (UTUC) in cell blocks with small distorted tissue fragments can be challenging; interobserver agreement is poor among pathologists. Mitotic figure (MF) counting along with nuclear features is important in grading these tumors. We evaluated the use of the mitotic-specific marker phosphohistone H3 (PHH3) as an adjunct to hematoxylin and eosin (H&E) stain for grading UTUC in cell blocks. Methods: Formalin-fixed, paraffin-embedded tissues from the cell blocks of 61 UTUC were stained with H&E and PHH3 antibody. The grading of tumors was performed independently by 3 pathologists, on both H&E-stained and PHH3 plus H&E-stained slides. The grading system used was the 1973 WHO 3-point grading system. Gradings were compared by all the pathologists for H&E staining versus PHH3 plus H&E staining with the Stuart-Maxwell test of marginal homogeneity that accounts for the matched data. Results: The average pairwise agreement by H&E alone was 55%, and 80% by PHH3 plus H&E. Conclusion: By adding PHH3 immunostain to the H&E, the agreement in grading the carcinomas among the 3 pathologists improved dramatically. PHH3 immunostain may play an important role in grading UTUC in small cell block samples.