Diphenylhydantoin (phenytoin) is a widely used drug with multiple adverse effects, several of them cutaneous. Prenatal exposure to hydantoins may result in a spectrum of structural, developmental, and behavioral changes known as the fetal hydantoin syndrome (FHS). A patient with the dysmorphic characteristics of FHS had unusual hyperpigmentation of several fingernails, a finding which has not been described in other patients with FHS. We propose a mechanism for the dysmorphic action of phenytoin on the developing fetus, consonant with the known effects of hydantoins on collagen metabolism.