Microsomal epoxide hydrolase gene (EPHX1) is polymorphic and encodes an enzyme involved in both the activation and detoxification of several tobacco carcinogens. Therefore, a contribution of EPHX1 enzymatic activity on lung cancer risk is possible. A genetic component of early-onset lung cancer has been suggested but variations in enzyme activity and polymorphisms in EPHX1 have seldom been studied in young patients with lung cancer. Primary lung cancer cases of both sexes and under age 45 at diagnosis were considered for this study. Controls fulfilled the following criteria: over 60 years old, smoking history of at least 40 years, no malignancies. Because of these criteria, they are referred to as super controls. The polymorphisms at exons 3 (Tyr113His) and 4 (His139Arg) as well as at the 5'-UTR-290T/G of the EPHX1 gene were genotyped by minisequencing. The association of these three polymorphisms with the development of early-onset lung cancer and the group of the super controls was evaluated by means of 2x2 tables using Yate's X(2) test or Fisher's exact test. Overall, data were obtained from 42 cases and 72 super controls. There was a significant association between early-onset lung cancer and the presence of the EPHX1 exon 4 variant (OR=3.33, 95% CI=1.50-7.41). This was confirmed at the phenotypic level when the data of both patients and super controls were stratified according to the predicted enzymatic activity (X(2) for linear trend=7.23, p=0.007). This analysis of lung cancer in subjects under age 45 supports the hypothesis that EPHX1 polymorphisms may have a role in cancer susceptibility in this age group.