Background: This study was planned to measure bone morphogenetic protein 2 (BMP-2) levels in the endometrial samples of unexplained infertility (UEI) cases who had a history of unsuccessful in vitro fertilization - embryo transfer (IVF-ET) and therefore decided to have frozen-thawed embryo transfer (FET). Methods: Thirty patients diagnosed with UEI and decided to transfer frozen-thawed embryo transfer were included in the study. The UEI patients were selected among the patients whose previous IVF-ET trials were unsuccessful and were scheduled for FET this time. Thirty participants in the control group were selected among patients diagnosed with polycystic ovary syndrome (PCOS) and scheduled for FET. Serum total testosterone, fasting insulin, follicle-stimulating hormone and luteinizing hormone levels were measured. Homeostatic model assessment - insulin resistance (HOMA-IR) Formula was used for calculating insulin resistance. Endometrial samples were collected by pipelle during oocyte pick-up, washed three times with a sterile saline solution to remove blood and transferred into RNA stabilization buffer until analysis. Endometrial BMP-2 concentrations were measured by enzyme-linked immunosorbent assay. Results: Serum total testosterone, insulin levels and HOMA-IR of patients in the control group were significantly higher than the UEI group. BMP-2 levels in the endometrial supernatants of UEI patients were found to be about 2 times lower than the patients in the control group (984.5 (350) pg/mg wet tissue vs. 1720 (318) pg/mg wet tissue, p 0.000, Z = –6.6). A strong and significant positive correlation was found between endometrial thickness, estradiol levels measured on HCG day and BMP-2 levels. A positive and significant correlation was found between endometrial BMP-2 levels and serum luteinizing hormone (LH), fasting insulin, glucose, testosterone, HOMA-IR and mature oocyte counts. Conclusion: BMP-2 is associated with UEI and mechanistically it may be useful to study this further to determine is this is causal or merely a biomarker.