The NQO1 locus on chromosome 16q2.2 encodes NAD(P)H:quinone oxidoreductase, an enzyme implicated in detoxication and protection against redox cycling. Two alleles have been identified in the human population, the rarer one, termed the null-allele, coding for a nonfunctional enzyme. Since lack of NQOR activity has been suggested to increase susceptibility to certain cancers, the distribution of the two alleles was determined by polymerase chain reaction-restriction fragment length polymorphism analysis in patients with renal cell carcinoma (n = 131) and urothelial carcinoma (n = 99) compared with a normal population (n = 260). Allele distribution in the normal population followed a Hardy-Weinberg distribution with frequencies of 0.867 for the major allele and 0.133 for the null-allele. Increased frequencies of the null-allele were found in the tumour patient groups (0.191 and 0.182, respectively) due to an increased number of both homo- and heterozygotes. The odds ratios for homozygous null-allele vs. wild-type genotypes were 1.7 and 3.6 for renal cell carcinoma and urothelial carcinoma, respectively. These data are compatible with the assumption that diminished activity of NQOR in some individuals increases susceptibility to certain cancers.