Epidemiological studies have led to the suggestion that a genetic basis may exist in the individual variation in predisposition to cancer. Interindividual differences in human toxicological response to carcinogenic exposure have been attributed to heritable polymorphisms in metabolism, namely glutathione S-transferases (GSTs) coding for enzymes that are known to be detoxifiers of carcinogens. Within the human GST mu class, there is a specific isozyme that is frequently lacking. To check whether or not this association exists in the Portuguese population with lung cancer, we used polymerase chain reaction (PCR)-based genotyping to examine GSTM1 polymorphism (nulled and non-nulled) in 84 individuals as a control healthy population and a group of 98 lung cancer patients. In this study we were able to find a frequency of the GSTM1 phenotype among our healthy control subjects consistent with earlier genotyping studies in other Caucasoid populations. For the group of individuals with lung cancer as a whole, or in subsets of histological subtypes, our data for the Portuguese population did not show a positive correlation between the null allele and this neoplasm. In contrast, we found a slight increase in the frequency of the wild-type allele in our lung cancer group.