Many diseases can result in nail thickening, such as psoriasis, onychomycosis, pityriasis rubra pilaris, pachyonychia, and ichthyosis. In this report, the authors discuss such a condition, called onychogryphosis. Onychogryphosis is a thickening of the nail plate, with associated gross hyperkeratosis and increased curvature. It can be oyster-like or resemble a ram's horn. Its irregular surface is marked by longitudinal and transverse striations, the latter of which is more frequent. The diagnosis of onychogryphosis is difficult to identify in the early stages because hypertrophy of the nail plate is the earliest manifestation, with the more classical features appearing later. Onychogryphosis is common on the great toenail of elderly patients and can be associated with self-neglect, homelessness, dementia, old age, trauma, peripheral circulation disorders, diseases of the central nervous system, and foot abnormalities, such as hallux valgus, elephantiasis, long-standing poor personal care, hyperuricemia, and pressure from improper footwear. All fingers and toenails can be affected in hereditary onychogryphosis. In infants and children, the presence of onychogryphosis can be accompanied by congenital malalignment of the great toenail.