Microsomal epoxide hydrolase (mEH) is involved in the metabolism of highly reactive epoxide intermediates. The frequency of the genetic polymorphims is expected to vary among different ethnic and racial groups. The aim of this study is to investigate the genotype frequencies of mEH in Turkish bladder carcinoma patients (n:140) and healthy subjects (n: 114). According to data; genotype distribution in healthy controls and in bladder cancer patients detected by Tth111I digestion was found to be as follows: YY 66.7%, HH 11.4%; HY 21.9%; and YY 54.3%, HY 45.7%; HH not detected respectively. Our study provides aframe work for future studies concerning the role of geno-environmental interaction in the genesis of disease and in particular the role of mEH as a susceptibility for cancer.