Human serum paraoxonase 1 (PON1, EC 126.96.36.199) is a high density lipoprotein (HDL)-associated antioxidant enzyme that not only decreases oxidative stress, but it is also implicated in development of many cancers. Genetic information provides a means of identifying people who have an increased risk of cancer, thus this knowledge of cancer genetics helps to identify the ability to characterize malignancies leading to the development of new therapeutic approaches. Because of this reason, in this preliminary study we aimed to investigate the role of human serum PON1 enzyme activity and phenotypic distribution in 32 breast cancer (BC) patients (age range 28-82) and 35 cancer free (CF) control group (age range 21-67). PON1 enzyme was prepared from the serum pool of BC patients using hydrophobic interaction chromatography on L-tyrosine-9-aminophenanthrene-coupled Sepharose 4Bgel. The PON1 enzyme activity towards paraoxon substrate was quantified spectrophotometrically. The basal activity of PON1 was statistically decreased in cancer cases compared to the control group. In addition, individuals were classified according to phenotyping of human PON1 Q and R types. In the cohort of BC patients, an increase in the frequency of the PON homozygote Q (AA) genotype was observed (31% in the BC group versus 14% in the CF controls). The frequency of the PON heterozygote QR (AB) genotype was 34.5% in the patients with BC and 37% in the CF group. The same trend was observed in PON homozygote R (BB) genotype frequency (BC cases 34.5% versus controls 49%). We determined that the kinetic parameters of the purified enzyme by Lineweaver-Burk method. We obtained K-m and V-max values of 0.227mM and 62 U/mL min for the BC enzyme, compared with 0.775 mM and 206 U/mL min for the CF control enzyme. As a conclusion, it is clear from our results that while the PON1 AA allele frequency in BC cases is much higher, that of BB allele is much lower, in comparison with the control group. The most significant finding of this study is AA allele activity which is low in BC cases was found high. We concluded that decreased AA allele PON1 activity might have a relation with BC.