To investigate the association of CYP 17 -34T/C polymorphism with insulin resistance (IR) in Thai polycystic ovary syndrome (PCOS). A cross-sectional study was performed on 210 Thai women diagnosed with PCOS. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was used to analyze CYP17 polymorphism at -34T/C. Student's t-test was used to compare the mean of normally distributed variables between A1/A1 and A2/X. Chi-squared or Fisher's exact tests and OR were used to analyze the association at P < 0.05. Out of 210 cases, PCR-RFLP was successful in 199. Mean patient age was 24.4 ± 4.7 years, with body mass index 25.2 ± 6.3 kg/m(2) . There were 65 and 134 women in the A1/A1 genotype group and A2/X genotype group, respectively. The A2/X genotype group was statistically significantly younger and had a strong trend toward overweight/obesity compared with the A1/A1 genotype group. The prevalence of IR according to different methods varied from 15.4% to 70.8% and was not different between the two groups. On subgroup analysis, in the overweight/obese PCOS group, the A2/X genotype was not associated with any indices of IR. No significant association between CYP17-34T/C polymorphism and IR was found in Thai PCOS women, although the A2/X genotype group was statistically significantly younger than the A1/A1 genotype group.