OBJECTIVE: To study the possible relationship between plasminogen activator inhibitor-1 (PAI-1) and thrombomodulin expression on deciduas and recurrent firsttrimester miscarriages of unknown etiology. STUDY DESIGN: The miscarriage group consisted of 16 women who miscarried during the first trimester of gestation, and controls consisted of 16 healthy women who had electively terminated their pregnancies during the first trimester of gestation. The abortion material was processed, and specimens taken were studied using immunohistochemical methods. Specimens were taken from decidua basalis and decidua parietalis. Monoclonal antibodies were used against PAI-1 and thrombomodulin. The results were statistically analyzed with Mann- Whitney test. RESULTS: PAI-1 and thrombomodulin expression was detected in decidua basalis and parietalis from both the miscarriage group and controls. No statistically significant difference was detected in the expression of PAI-1 and thrombomodulin between the miscarriage group and controls (p < 0.05). CONCLUSION: Our study was a first attempt to investigate the possible involvement of decidual thrombomodulin in recurrent pregnancy loss of unknown etiology. Due to lack of published data, further investigation is needed. © Science Printers and Publishers, Inc.