Although endometriosis could be associated with certain epithelial ovarian cancers and elevated serum cancer antigen (CA) 19-9 levels, it is difficult to diagnose endometriosis using this marker. We report an unusual case of endometriosis that evolved into endometrioid carcinoma in an asymptomatic 38-year-old woman with elevated CA 19-9 levels who presented to our clinic. Transvaginal ultrasound and computed tomography of the abdomen and pelvis revealed endometriosis of the right ovary. A laparoscopy was performed to evaluate for chocolate cysts. However, based on the postoperative pathological assessment, she was diagnosed with endometrioid carcinoma associated with endometriosis and was reoperated for surgical staging. This case indicates that sudden elevation of CA 19-9 level might be a sign of malignancy, small asymptomatic endometriosis should not be ignored, and incidental diagnosis of ovarian cancer should be carefully assessed by surgical staging.