The pathologic findings in placentas from ten multiple gestations complicated by the so-called vanishing twin phenomenon were studied to confirm the ultrasonographic evidence. Five pregnancies resulted from in vitro fertilization and embryo transfer, and five conceptions were spontaneous. The pregnancies were studied by repeat ultrasound examinations between five and 12 weeks' gestation. First-trimester bleeding was the only clinical sign of this phenomenon. Postpartum evidence of the vanishing twin phenomenon was found in five cases. Morphologically, the lesions were characterized by well-delineated plaques of perivillous fibrin deposition, associated in one case with embryonic remnants. This focal degenerative change of the placental mass, which also exists in about 25% of placentas from uncomplicated term pregnancies, may be the only clue to the disappearance of one conceptus.