Ovarian endometrioma accompanying with elevated levels of serum tumor antigens and free peritoneal fluid may be misdiagnosed as ovarian cancer. A 40 year-old nulliparous woman was referred to our clinic with 15x15x12.4 cm and 6x4x3.7 cm hypoechoic right adnexal and 7.3x3.8x3.2 cm hypoechoic left adnexal mass accompanying with excessive free fluid at pouche of douglas and elevated levels of serum CA125 (143.72 U/mL) mimicking ovarian cancers. Laparoscopy was performed. Ruptured right ovarian endometrioma and unruptured bilateral ovarian endometrioma adherent to sigmoid colon and omentum were observed. Right adnexectomy, left ovarian cystectomy and bridectomy was performed. Histological examination confirmed the diagnosis of endometrioma. Adnexal mass with free peritoneal fluid and high levels of tumor antigen CA125 mimicking ovarian cancer has been rarely diagnosed as endometrioma.