Purpose: Diagnosis of pelvic inflammatory disease (PID) is based on clinical history and examination; however, it may be difficult to distinguish from other disease entities. Multidetector computed tomography (MDCT) is a useful radiologic modality, which can be performed in an emergency department (ED). The aim of the current study was to clarify the MDCT characteristics of clinically severe PID by comparison of patients with clinically more severe and less severe forms of PID. In addition, we evaluated the independent predictors of MDCT findings in the severe PID group. Methods: We conducted a retrospective study of female patients with symptoms and signs of PID who visited the ED at our institution during a five-year period. Patients who underwent abdominal MDCT and were diagnosed with PID were retrospectively enrolled in the study. For determination of CT characteristics, each patient was evaluated for pelvic edema, amount of ascites, Hounsfield units (HU) of ascites, salpingitis, oophoritis, intrauterine devices, peritoneal fat infiltration, cervicitis, abnormal endometrial enhancement, tubo-ovarian abscess, adjacent bowel wall thickening, localized ileus, and perihepatitis. Patients were divided into two groups: clinically more severe and less severe forms of PID. Patients having the clinically more severe form of PID were defined as follows: (1) initial body temperature over 38.3?C, (2) initial systolic blood pressure < 90 mmHg, (3) intractable abdominal pain, or (4) uncontrollable nausea or vomiting despite medication. We compared data between the two groups. Results: A total of 136 patients were enrolled in this study. Thirty eight patients had the clinically more severe form (28%) and 98 patients had the less severe form (72%). In comparison with subjects in the group having the less severe form, the amount of ascites (p<0.001), salpingitis (p<0.05), and tubo-ovarian abscess (p<0.01) differed statistically between the groups. The HU value of ascites in the more severe group, 19.56±11.14 HU, was significantly greater, compared with that of the group having the less severe form. Results of multivariate logistic regression analysis revealed an association of the amount of ascites, a high HU value, and atubo-ovarian abscess with increased odds of the more severe form (adjusted OR 3.25, 95% CI 1.01-10.45; adjusted OR 5.84, 95% CI 1.80-18.95; and adjusted OR 8.42, 95% CI 1.73-40.96, respectively). Conclusion: Patients with clinically more severe PID show more clinically important findings on MDCT, such as a greater amount of ascites, higher HU value of ascites, and tubo-ovarian abscess. Leukocytosis, increased neutrophil percentage, and elevated CRP were observed in patients with severe PID.