The human population consists of slow and fast aaetylators. To test the hypothesis that slow aoetylators are predisposed to bladder cancer caused by exposure to aromatic amines, we performed a retrospective study, phenotyping cancer patients. Bladder cancer patients were divided, based on interview data, into two groups: those with and without occupational exposure to aromatic amines. The nonexposed group had a distribution of slow acetylators (60.5%) similar to that in the general Polish population. (A group of controls was 45.4% slow acetylators.) The exposed group was predominantly slow acetylators (87.6%). A prospective study in a currently healthy ocoupationally exposed cohort has been initiated to investigate possible change in acetylator phenotype with disease or therapy onset. Cohort members have been typed for acetylator status; a tumor marker (to identify bladder cancer at a preolinical stage) and acetylator status will be measured periodically.