The identification of genetic traits that predispose individuals to environmentally induced cancers is one of the challenges in the assessment of individual cancer risk. For this reason, individual variations in the expression of enzymes involved in biotransformation reactions have been extensively studied. One such polymorphic enzyme is GSTM1, which belongs to the class Mu of glutathione S-transferases (GSTs), and is only expressed in 55-60% of Caucasians. Previous data suggest that smokers lacking GSTM1 activity may be at greater risk of developing lung cancer. In this study, we used a polymerase chain reaction-based method to examine this issue in a Finnish study population. We found that 44% of a control group of 142 individuals lacked the GSTM1 gene, i.e. they had the GSTM1 null genotype; the rest were either homozygous or heterozygous for the expressed GSTM1 alleles. In a group of 36 patients with non-neoplastic pulmonary diseases, an identical distribution was observed. However, among 138 lung cancer patients the distribution of the GSTM1 genotypes deviated from that found in the healthy controls (53% nulled; odds ratio 1.5, 95% confidence interval 0.9-2.3). Furthermore, when the lung cancer patients were analysed by tumour type, a statistically significant increase in the GSTM1 null genotypes (62%; n = 71) was seen in the squamous cell carcinoma group, with an odds ratio of 2.1 (95% confidence interval 1.2-3.8). These data support the suggestion that GSTM1 null genotype may act as a risk modifier in lung cancer.