Schistosoma haematobium infection is strongly associated with urinary bladder cancer. Although numerous explanations have been proposed for this association, the nature of this relationship remains unresolved. This paper explores the hypothesis that inflammation and elevated cell proliferation play a major role in the development of bladder cancer in infected patients, possibly by increasing the level of genetic instability in the urothelium. The paper details in vivo and in vitro studies being done in our laboratories to test this hypothesis. These studies include population studies in which chromosomal breakage in the bladder of infected individuals is assayed using the micronucleus (MN) test on exfoliated urothelial cells. The approach also includes parallel studies in Vancouver with patients with long-term catheter drainage, a population with many similarities to schistosomiasis patients. In the in vitro studies we are co-incubating bladder cells with activated neutrophils or experimental conditions simulating inflammation. These studies show that inflammatory cells when activated can induce micronuclei in bladder cells and that this response is associated with loci on chromosome 11, a chromosome commonly altered during bladder carcinogenesis. A final approach being used is to assay chromosomal change (MN frequencies and numerical chromosome alterations) and level of proliferation (expression of proliferating cell nuclear antigen) in archival biopsies from schistosomiasis patients. Preliminary results show that a dysregulation of cell proliferation is occurring during cystitis in these patients. The extent to which this alteration affects the level of chromosomal breakage is yet to be determined.