To assess the risk factors for antibiotic therapy failure and to predict which patients will require surgical drainage for tubo-ovarian abscess. Methods: We collected data from patients by ICD-9 codes starting with 614.2. We extracted data regarding background information, clinical presentation, laboratory parameters, and ultrasonographic findings. Patients responded to antibiotics alone were compared with patients required surgical drainage. Relative risk of surgical drainage was estimated with logistic regression model. Result: A total of 126 cases of tubo-ovarian abscess were evaluated, of which 92 were successfully managed with antibiotic therapy alone and 34 required surgical drainage. Age, multiparity, intrauterine device use, fever, maximal white cell count and abscess size were identified to be significant risk factors associated with the need for surgical drainage. The adjusted relative risks of surgical drainage were 2.250 for abscess size ≥8 cm, and 3.162 for fever on admission. The duration of hospitalisation was increased by 23.8% for abscess size ≥8 cm and by 23.7% for fever on admission. Conclusion: Larger abscesses are associated with increased risk of surgical drainage. However, additional research is required to determine the optimal treatment for large abscesses. It is reasonable to try antibiotic therapy in clinically stable patients irrespective of abscess size.