Dipeptidyl peptidase-4 (DPP-4) plays a role in metabolic and inflammatory diseases. Increased adenosine deaminase (ADA) has been suggested to induce insulin resistance and inflammation. We measured serum DPP-4 and ADA activities. Serum ADA activity was significantly higher in PCOS group (p=.006), whereas there was no difference in serum DPP-4 activity between the groups (p>.05). When the study subjects were divided into four groups in terms of obesity; an increasing trend in serum ADA activity between the groups was observed and ADA activity was significantly higher in overweight and obese patients with PCOS than nonobese controls (p=.016), there were no significant differences between the other groups (p>.05). A positive correlation was found between ADA and BMI in the whole group (p=.022). Multivariate regression analyses revealed that significant determinants were diastolic blood pressure, ADA, and the presence of PCOS for DPP-4 (R-2=0.344, F=9.079, p<.001); the presence of PCOS and DPP-4 for ADA (R-2=0.123, F=6.302, p=.003). We demonstrated increased serum ADA activity as well as its association with obesity in PCOS, while there was no change in serum DPP-4 activity in women with PCOS.