Abstract： Objective To investigate the relationship between activity of plasma platelet-activating factor acetylhydrolase (PAF-AH)and insulin resistance (IR) in patients with polycystic ovary syndrome (PCOS). Methods From Oct. 2006 to Jan. 2008, 105 PCOS patients undergoing treatment in Department of Obstetrics and Gynecology of Second Hospital affiliated to West China University were enrolled in the study, among 53 cases with homeostatic model assessment IR (HOMA-IR) exceed or equal 2.77 as IR-PCOS group and 52 cases with HOMA-IR less than 2.77 as non IR-PCOS group. In the mean time, 53 infertile women due to fallopian tube or husband factors were chosen as control group. Plasma PAF-AH activity, high-density lipoprotein-associated PAF-AH (H-PAF-AH) activity, low-density lipoprotein-associated PAF-AH (L-PAF-AH) activity, the ratio of L-PAF-AH to H-PAF-AH activity were measured and compared among three groups. The method of Pearson correlation analysis and stepwise multiple regression analysis were used to study the relationship between the ratio of L-PAF-AH to H-PAF-AH activities and hormonal and metabolic parameters. Results (1) Plasma PAF-AH activity [ (0.055±0.012) mmol· L~(-1)·min~(-1)], L-PAF-AH activity [(0.052±0.012) mmol·L~(-1)·min~(-1)], and the ratio of L-PAF-AH to H-PAF-AH activity (23±6) in the IR-PCOS group were significantly higher than those at control group [(0.050±0.009) mmol·L~(-1) ·min~(-1), (0.047±0.009) mmol·L~(-1)·min~(-1) and (18±4)] and non-IR-PCOS group [(0.050±0.0012) mmol·L~(-1) ·min~(-1), (0.048±0.012) mmol·L~(-1)·min~(-1) and (18± 5 ), P <0.05 ]. The ratio of L-PAF-AH to H-PAF-AH activities at IR-PCOS group was also significantly higher than those at Non IR-PCOS and control group after correction for body mass index (P<0.01). (2) Pearson correlation analysis showed that the ratio of L-PAF-AH to H-PAF-AH in PCOS patients was positively correlated with atherogenic index, age, body mass index, waist-to-hip ratio, HOMA-IR, triglyceride levels, fasting insulin levels, fasting glucose levels, low density lipoprotein cholesterol and total cholesterol (r=0.644, 0.247, 0.296, 0.212, 0.356, 0.587, 0.377, 0.375, 0.292 and 0.199, respectively, P<0.05), and negatively correlated with high density lipoprotein cholesterol (r=-0.510, P<0.05). The multiple stepwise regression analysis showed that atherogenic index and triglyceride were significant variables for the ratio of L-PAF-AH to H-PAF-AH activity (standardized coefficient, β=0.515 and 0.201, respectively, P<0.05). Conclusion The elvated L-PAF-AH activity may be associated with IR in PCOS patients, and the increased ratio of L-PAF-AH to H-PAF-AH activities may be a potential marker of inflammation in the patients.