Background: Ultrafast magnetic resonance imaging (MRI) has been used successfully in antenatal diagnosis. Case: We report a case of cephalopagus conjoined twins in which ultrafast MRI clarified how the twins were united, especially the neuroanatomy, and helped with antenatal counseling. Conclusion: Ultrafast MRI can provide superior image quality to two-dimensional ultrasonography and should be considered an adjunct to ultrasound for antenatal characterization of some anomalies. (Obstet Gynecol 2000;95:1015-7. (C) 2000 by The American College of Obstetricians and Gynecologists.).