Scientific and ethical issues involved in the genetic screening of individuals with increased susceptibilities to developing occupationally related cancers were examined. Minimum scientific evidence required for the use of genetic screening tests were described. Difficulties associated with obtaining such minimum evidence were illustrated by examination of the genetic polymorphism associated with the activity of N-acetyltransferase, an enzyme associated with the activation and deactivation of carcinogenic arylamines, and the risk of bladder cancer among workers exposed to such chemicals. Drawbacks to the use of N-acetyltransferase polymorphism as a biological marker of susceptibility to cancer were discussed. These included unavoidable misclassifications resulting from poor correspondence between phenotype and genotype or intraindividual phenotype variability, variability in the data linking specific N-acetyltransferase polymorphisms with cancer risk dependent upon exposure and population, violation of the autonomy of workers, ethical considerations, and the potential for employment restrictions based upon prejudices such as gender, race, or ethnic group that may be associated with this polymorphism.