Objective: The marker for the early diagnosis of endometriotic cyst rupture is unknown. We report a preliminary study designed to evaluate the relationship between plasma D-dimer levels and endometriotic cyst rupture in clinical case series. Materials and methods: We reviewed the patients' records of endometriotic cyst rupture cases, and the background (i.e., age, body mass index, and parity) and preoperative laboratory assessments (i.e., white blood cell count, levels of serum C-reactive protein, serum CA125, and plasma D-dimer) of the patients were compared with those of unruptured cases. Results: Emergency surgery cases of endometriotic cyst rupture (n = 6) and planned surgery cases of unruptured endometriotic cysts as controls (n = 16) were reviewed. Backgrounds of the patients were not significantly different between the two groups. The plasma D-dimer level was significantly higher in the rupture cases (8.5 μg/mL vs. 0.20 μg/mL, p < 0.001). Differences in white blood cell count and serum C-reactive protein level, but not serum CA125 level, were found to be statistically significant between groups. Conclusion: An elevation of plasma D-dimer level is associated with endometriotic cyst rupture.