A case report of a primary interstitial ovarian pregnancy is presented. A 37-year-old married woman with two children after two Cesarean sections and a spontaneous abortion, with a contraceptive intrauterine device (IUD) inserted three years before, presented at five weeks plus five days amenorrhea with a positive pregnancy test and lower abdominal pain but with no vaginal bleeding. Her previous menstrual cycles had been regular. She was hemodynamically stable. On bimanual examination, the uterus was of normal size, and there was an approximate four-cm tender right adnexal mass. Serum beta-human chorionic gonadotropin (b-hCG) was confirmed positive. Ultrasound revealed a well-positioned IUD in the uterus and a right adnexal mass with normal vascular flow on Doppler, that contained a well-defined gestational sac, well-distinct from the quiescent hemorrhagic corpus luteum. There was no fetal node or cardiac activity or free fluid. The patient received four injections of methotrexate intramuscularly using the multidose regimen that involves the administration of methotrexate calculated according to body weight, alternated with 0.1 mg/kg of leucovorin calcium per os after 30 hours until the values of β-hCG had decreased by 15%. The patient's post-treatment period was uneventful with a full restoration of ovarian morphology and the complete absorption of the gestational sac. This case is the first where diagnosis was made by endovaginal sonography and treatment was made by multidose methotrexate. Spiegelberg criteria for the diagnosis of ovarian pregnancy are obsolete; new ultrasound and laboratory criteria are needed for a diagnosis as early as possible without the need of surgery.