The macroscopic and microscopic features of the urothelial response of the human urinary bladder to urethral catherisation are described. The catheter reaction is characterised by a predominantly eosinophilic inflammatory response producing, macroscopically, a papillary mucosal appearance termed polypoid cystitis. The severity of the epithelial inflammatory response correlates significantly with the duration of catheterisation. Urothelial dysplasia confined to the catheter reaction site was noted in 6% of cases. The possible implications of these findings are discussed.