Hepcidin, originally identified as a 25 amino acid peptide antibiotic produced in the liver, is a key regulator of iron balance and recycling in humans and mice. Closely related hepcidin genes and peptides also have been identified in other mammals, amphibians, and a number of fish species. We hypothesize that hepcidin, the iron-regulatory hormone in humans, may have evolved from an antimicrobial peptide in fishes. In this review we will highlight the evidence that indicates hepcidin evolved from an antimicrobial peptide to an iron-regulatory hormone in vertebrate evolution. This evidence includes the discovery of hepcidin as an antimicrobial peptide and iron-regulatory hormone, structural comparison of mammalian hepcidins and nonmammalian hepcidins, and the cellular and molecular evidence indicating that, while some fish hepcidins may serve only as antimicrobial peptides, other fish and amphibian hepcidins may function as iron regulators.