Introduction: The aim of this study was to determine whether there are quantitative variations in the numbers of mast cells (MCs) in bladder carcinoma tissue associated with the histopathological grading of tumours. Materials and Methods: Specimens of 56 bladder carcinomas were stained with toluidine blue and histologically staged using the Mostofi system. The MC counts were assessed within tumour tissue and lamina propria of the bladder. The MCs in the adjacent 'normal bladder tissue' of 14 specimens from patients who underwent cystectomy and of 10 specimens from patients having interstitial cystitis were also determined. The results were analyzed by using the Student t test, the Wilcoxon signed-rank test, and the Spearman correlation (r(s)). Results: The mean MC concentration was 0.57 within the tumour tissue and 3.36 in the lamina propria. The difference between the two MC groups was statistically significant (p < 0.0001); there were no correlations between mean MC counts in tumour tissue and histopathological tumour grade and between mean MC counts in lamina propria and histopathological tumour grade. Statistically significant differences were also observed between tumour group and interstitial cystitis group ( p = 0.029) and between tumour group and normal 'adjacent' tissue group ( p = 0.037). Conclusions: Our findings suggest that MCs aggregate in small numbers in the lamina propria of bladder carcinomas and that the MC count is related to tumour differentiation. The number of MCs may be a useful prognostic indicator in patients with bladder carcinoma. Copyright (C) 2004 S. Karger AG, Basel.