Polycyclic aromatic hydrocarbons, found in cigarette smoke, food and industrial materials, are potential human carcinogens. Deficiency of detoxifying enzymes, such as glutathione transferases, may affect the metabolic fates of these chemicals and raise cancer risks in exposed individuals. The GSTM1 null genotype is a common form of glutathione transferase deficiency. Because knowledge of its ethnic distribution would be useful in epidemiologic studies, we measured the frequencies of the GSTM1 null genotype among healthy blacks, whites, Asian Indians, Chinese, Japanese, Koreans, Filipinos, Samoans and Hispanics. Rapid genotyping was done by use of a PCR assay, with dried blood spots on blotter paper as DNA templates. The frequency of the null genotype ranged from 0.31 among blacks to 0.88 among Samoans. The PCR assay was also applied to a pilot study of 114 bladder cancer cases from Kaiser Permanente Medical Center, Harbor City, California. DNA for these cases was obtained from paraffin-embedded surgical specimens. The overall odds ratio for bladder cancer with the GSTM1 null genotype was 1.4 (95% confidence interval 0.94–2.1), Indicating no statistical difference in null genotype frequencies among bladder cancer patients compared to a healthy population. Large epidemiologic studies, which can be accomplished with dried blood spots or paraffin-embedded tissue specimens, may be useful for further assessment.