A 13-year-old girl presented with non-specific symptoms of peritonitis, low grade fever and missed her normal menses. Computed tomography of her abdomen and pelvic cavity showed omental cake and a generalised thickened peritoneum. An elevated serum cancer antigen-125 (CA-125) level of 1248.5 U /ml was also noted and a diagnosis of peritonitis carcinomatosis was first suspected. Diagnostic laparoscopy revealed multiple tubercles over the whole peritoneum and the pathology report described granulomatous nodules with giant cells and epithelioid cells. Culture of the ascitic fluid revealed Mycobacterium tuberculosis which resulted in a final diagnosis of tuberculous peritonitis. The patient's fever and abdominal distension gradually subsided after anti -tuberculosis treatment. The serum CA-125 level also decreased significantly to 10.2 U/ml after treatment. Conclusion:cancer antigen-125 levels may serve as a potential follow-up marker of disease activity and treatment response in tuberculous peritonitis. ; 附設醫院小兒部 ; 醫學院附設醫院 ; 期刊論文