Pachyonychia congenita is an uncommon autosomal dominant disorder with variable expression. Symmetrical nail hypertrophy, present in nearly all cases, is accompanied by dyskeratosis and dysplasia of other ectodermal tissues. This article reviews the genetics, clinical manifestations, histopathology, and treatment of pachyonychia congenita. Many clinical features have been reported in association with this syndrome. From a review of the literature, we propose criteria for the diagnosis of pachyonychia congenita using the more important of these clinical manifestations.