Background: Rudimentary horn pregnancy is a rare form of ectopic pregnancy, and rupture of the pregnant rudimentary horn in the second trimester is the usual presentation. Most cases are diagnosed at laparotomy for hemorrhagic shock, with increased maternal morbidity and, at times, mortality. Although the diagnosis prior to rupture is difficult, early detection is essential for timely management. Among various investigative methods, such as ultrasound scanning, computed tomography scanning, magnetic resonance imaging, and laparoscopy, ultrasonography plays an important role in early diagnosis of this condition and in planning management. Case: A 23-year-old woman, gravida two, para one, A0, in the fifth month of her pregnancy was referred by a private practitioner after routine ultrasonography revealed a live extrauterine pregnancy. Her general condition was good. A repeat ultrasound at the Postgraduate Institute of Medical Sciences, Rohtak, Haryana, India, showed a rudimentary-horn pregnancy of 15 weeks gestation and a normal uterus on her right side. Laparotomy, followed by excision of the rudimentary horn was performed. Results: The patient's postoperative period was uneventful. Conclusions: Although diagnosis prior to rupture is difficult, early detection of rudimentary-horn pregnancy is essential to avoid a fatal outcome. One should suspect this rare entity if a patient is complaining of lower abdominal pain in early pregnancy. Early ultrasonography in such cases should be performed, because it plays an important role in diagnosing rudimentary-horn pregnancy especially in the prerupture state, as was seen in this case. our case. ( J GYNECOL SURG 29:140)