Although the phenomenon of the "vanished twin" has been noted repeatedly through the use of ultrasound, no confirmatory histologic evidence has been presented previously. This has raised questions concerning the validity of the vanishing twin syndrome. In the following case, a triplet intrauterine pregnancy was diagnosed ultrasonographically four weeks after in vitro fertilization, but only a single fetus and placenta were delivered at term. Careful examination of the placenta revealed histologic evidence of the vanished twin. This evidence consisted of a chorion-lined sac containing amorphous material, surrounded by degenerated chorionic villi juxtaposed against a normal amniochorionic membrane.