The fingernails of twenty-four patients with classical adult (type 1) pityriasis rubra pilaris were examined for changes in nail morphology and a comparison was made with the fingernail morphology of twenty-seven consecutive psoriatic patients with nail changes. Distal yellow-brown discoloration, subungual hyperkeratosis, nail plate thickening, and splinter hemorrhages indicate a diagnosis of type 1 pityriasis rubra pilaris rather than psoriasis, while onycholysis (particularly marginal), salmon patches, small pits, and larger indentations of the nail plate indicate a diagnosis of psoriasis. Histology demonstrated that a nail biopsy would be a useful diagnostic procedure if nails were involved in isolation but provided no additional diagnostic features to those found from biopsy specimens of involved skin in the two conditions. The similarity in nail morphology between the type 1 pityriasis rubra pilaris patients and five patients with chronic erythroderma resulting from Sézary syndrome indicated that these changes may represent a nonspecific reaction pattern that may result from prolonged erythema of the proximal nail bed and matrix. The rough nails (trachyonychia) described in advanced Sézary syndrome were not observed in any of our patients.