SummaryThe majority of primary bladder neoplasms are known to arise within the mucosa around the ureteric orifices and bladder base. This may be due to the mucosa in this area being more susceptible to carcinogens than other areas of the bladder. Deficiency in the nucleotide salvage pathway enzyme thymidine kinase (TK), and especially its TK 1 isozyme, has been shown to predispose cell lines to increased mutagenesis. Total TK and TK 1 activities were measured in mucosal samples taken adjacent to the ureteric orifices and dome in 32 normal bladders and both total TK and TK 1 were shown to be significantly decreased in the mucosa adjacent to the ureteric orifices.This may explain why primary bladder neoplasms occur more commonly in this site.