It has been suggested that expression of the P-glycoprotein transmembrane efflux pump (PGP), encoded by the multi-drug resistance (MDR1) gene, may play a role in the protection of epithelial tissues from a variety of local and systemic toxins. We report that in approximately 50% (6/11) of the population, MDR1 messenger RNA levels in the normal urinary epithelium are comparable to those found in the highest expressing tissues in the body, and suggest a role for PGP in the normal bladder urothelium. MDR1 mRNA levels in the normal urothelium do, however, vary over a 60-fold range between individuals, and furthermore are uniformly significantly lower (about 6-fold, P</=0.01) in all low-grade bladder carcinoma samples. On the basis of these observations we conclude that low MDR1 mRNA levels are a uniformly consistent characteristic of low-grade bladder tumours, and hypothesize that high MDR1 expression plays a role in protection of the normal bladder from carcinogen exposure, and that individuals with low normal bladder MDR1 mRNA levels may consequently be at an increased risk of developing bladder cancer. Furthermore,the low level of MDR1 expression generally found in low grade superficial tumours may predispose them to additional carcinogen exposure and in this way contribute to possible tumour progression. In addition, MDR1 mRNA levels were observed to be elevated in a significant proportion (approximately 25%, 8/30, P = 0.015) of high grade tumours compared to low grade samples, and may therefore represent a marker of bladder tumour progression.