Recently, it was reported that gene polymorphism for microsomal epoxide hydrolase (mEPHX), an enzyme involved in the first-pass metabolism of epoxide intermediates, was associated with susceptibility to emphysema. This association was examined in a Japanese population, performing polymerase chain reaction (PCR)-based direct sequencing and restriction fragment length polymorphism assays for variant forms of mEPHX. The subjects consisted of 79 smokers with moderate to severe emphysema diagnosed by lung computed tomography scans, 58 smokers without emphysema, with a comparable smoking history, and 114 consecutive subjects who undertook annual health checkups. The allele frequency of exon 3 Tyr113 to His113, which was reported to confer slow mEPHX activity, was substantially higher in the population control group compared with that of the Caucasian control subjects in a previous study. However, neither the genotype distribution of exon 3, nor that of exon 4 His139 to Arg139, was significantly different between the two groups of smokers. These data indicate that the gene polymorphism for mEPHX is not associated with susceptibility to emphysema in the Japanese population. The discrepancy between the two studies may be explained either by racial difference or by the selection bias of subjects in the previous study, which examined those who had only mild to moderate emphysema with lung cancer or those who were clinically diagnosed as having chronic obstructive pulmonary disease.