The acetylator phenotype of 26 bladder cancer patients and 26 controls was determined by the sulfamethazine method to evaluate whether patients with the slow acetylator phenotype have a greater susceptibility for bladder cancer. This hypothesis has been suggested by experimental animal and human epidemiological observations. Of the 26 bladder cancer patients 12 (46 per cent) had the slow acetylator phenotype compared to 18 of 26 controls (69 per cent). Within the bladder cancer group there was no striking excess of the slow acetylator phenotype when subgrouped by occupational and smoking history. Our results show no significant association between the slow acetylator phenotype and human bladder cancer.