A large excess of patients with bladder cancer who have previously been exposed to N-substituted aryl compounds as a result of the production of dyestuff intermediates have the slow phenotype of the enzyme N-acetyltransferase. Among bladder-cancer patients in general, those presenting with T3 or T4 disease or carcinoma-in-situ also show an excess of the slower subtypes. Either N-substituted aryl compounds more frequently produce tumours with this invasive potential if linked with slow acetylation or slow acetylators are more susceptible to tumour production when exposed to some N-substituted aryl compounds. It is suggested that acetylator status could be used to identify susceptible individuals in potentially hazardous occupations.