The authors present a case of carcinoma of unknown primary site detected accidentally during laparoscopic surgery for ectopic pregnancy. A 36-year-old female patient (para 1, gravida 1) was referred to the present hospital with suspected ectopic pregnancy. Transvaginal ultrasound showed a right adnexal mass and echo-free space in the pelvic cavity. Her serum human chorionic gonadotropin level was 7,590 mIU/ml. The authors made a preoperative diagnosis of ectopic pregnancy and performed laparoscopic surgery. The ampulla of the right fallopian tube was enlarged and showed blood clotting. They noticed a white solid mass of size 1 cm on the peritoneum around the right pelvic ligament. Pathologic examination findings of the specimen were consistent with carcinoma. Immunohistochemical stains suggested high-grade neuroendocrine carcinoma. Routine metastatic workup could not reveal the original site. The final diagnosis was carcinoma of unknown primary site. The authors reviewed other case reports on neoplasms coexisting with ectopic pregnancy.