Purpose: This study aimed to evaluate the level of progesterone and progesterone-induced blocking factor (PIBF), an immune mediator, in non-obese patients with polycystic ovary syndrome (PCOS). Materials and Methods: Totally 72 patients were recruited into study and divided into 2 groups: The first group was patients diagnosed with PCOS (n = 36) and the second was the healthy control group (n=36). The diagnosis of PCOS was made according to Rotterdam diagnostic criteria. All patients were 18–35 years old and non-obese (body mass index (BMI) < 25 kg/m2). Follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol (E2), thyroid-stimulating hormone (TSH), prolactin (PRL), total testosterone, and dehydroepiandrostenedione sulfate (DHEA-S) levels were measured on the third day of the menstrual cycle. On the 21st day of the same menstrual period, fasting blood glucose, insulin, progesterone, and PIBF levels were measured. Results: Demographic and clinical characteristics of study participants were similar between the two groups. Serum FSH, E2, TSH, PRL, DHEA-S, total testosterone, fasting blood glucose, fasting insulin, homeostatic model assessment for insulin resistance (HOMA-IR), and hemoglobin A1c values were similar between the groups. Differences in LH, LH/FSH ratio, serum progesterone, and serum PIBF were statistically significant. Conclusion: Progesterone and PIBF levels decreased in non-obese PCOS patients. We suggest that even in the absence of obesity, which is the origin and enhancer of inflammation in PCOS, low PIBF as the underlying immunomodulator will drive complications. Amaç: Bu çalışmanın amacı obez olmayan polikistik over sendromlu (PKOS) hastalarda progesteron ve bir immün mediatör olan progesteron kaynaklı bloke edici faktör (PIBF) düzeylerini değerlendirmektir. Gereç ve Yöntem: Hastalar iki gruba ayrıldı. Birinci grup PKOS tanısı alan hastalar (n=36), ikincisi sağlıklı kontrol grubu (n=36) idi. Rotterdam tanı kriterlerine göre PKOS tanısı konuldu. Hastalar 18-35 yaşları arasındaydı, obez değildi (Vücut kitle indeksi (VKİ)