The pituitary gland is an endocrine organ that is developmentally derived from a fold in the oral ectoderm and a juxtaposed fold in the neural ectoderm. Here, we show that the absence of Vax1, a homeodomain transcription factor known for its role in eye and optic chiasm development, causes the rostral oral ectoderm to form an ectopic fold that eventually develops into a separate second pituitary with all the pituitary cell types and neuronal fibers characteristic of the normal pituitary. The induction of the second pituitary is associated with a localized ectopic expression of Fgf10, a gene encoding a growth factor known to recruit oral ectodermal cells into the pituitary. Interestingly, there are rare cases of pituitary duplications in humans that are also associated with optic nerve dysplasia, suggesting that VAX1 might be involved in the pathogenesis of this disorder.