Background/Aims: The purpose of this study was to identify predictive markers for tubo-ovarian abscess (TOA) through a comparison of clinical and laboratory data in patients diagnosed with pelvic inflammatory disease (PID). Methods: We reviewed the medical charts of 499 females who were admitted to hospital with clinical, surgical, imaging-based diagnoses of PID between 2001 and 2011. The patients were divided into the following two groups: (1) PID with TOA and (2) PID without TOA. Results: The TOA and non-TOA groups were comprised of 69 and 430 females, respectively. Mean age, history of intrauterine device (IUD) insertion and inflammatory markers, including erythrocyte sedimentation rate, C-reactive protein (CRP) and CA-125 levels, were higher in the TOA group than the non-TOA group. Independent factors that predicted TOA were older age, IUD insertion, increased CRP and CA-125, and chlamydia infection. CA-125 was found to have the highest predictive value for TOA. TOA size was associated with increased surgical therapy compared to patients with smaller abscesses. Conclusions: Increased age, IUD insertion, chlamydia infection, and increased CRP and CA-125 level were the independent factors predictive of TOA in acute PID. These predictive values will be expected to help decrease gynecological morbidity by early diagnosis and appropriate treatment of TOA.