Hepatic arylamine acetyltransferase phenotype has been suggested to be an important risk factor for urinary bladder carcinogenesis in individuals with known exposure to aromatic amines. This study was performed to evaluate the relative distribution of fast- and slow-acetylator phenotypes both in a population of men, 45 to 75 years of age, with a history of colorectal cancer and in a matched control group. Acetyltransferase activity was determined by administration of sulfamethazine and by subsequent analysis of blood and urine samples for N-acetylsulfamethazine and sulfamethazine using high-pressure liquid chromatography. The control group was composed of 28 slow-, two intermediate-, and 11 fast-acetylator individuals, while the group of patients with a history of cancer consisted of 20 slow-, three intermediate-, and 20 fast-acetylator phenotypes. This higher relative proportion of fast acetylators in the patients with a cancer history was highly significant and is consistent with the hypothesis that aromatic amines could play a role in the etiology of human colorectal cancer.