In the metabolism of the amino acid, tryptophan, certain products with the orthoaminophenol configuration are believed to act as topical carcinogens in the urinary bladder. In addition, a statistical relationship between cigarette smoking and bladder cancer has been established in recent years.Thirty metabolic studies are reported on six healthy male subjects when smoking and not smoking. Results revealed a consistent rise in carcinogenic metabolites of tryptophan when smoking (+ 50%), with a reciprocal fall in the end product, N'-methylnicotinamide (- 34%). Carcinogens fell and N'-methylnicotinamide rose when subjects stopped smoking. These metabolic studies confirm the statistical relationship between smoking and bladder cancer, and suggest that cigarette smoking blocks the normal metabolism of tryptophan, leading to the accumulation of carcinogenic metabolites.