In a clinical study, 27 of 31 cases of pityriasis rubra pilaris (PRP) had two or more of the following clinical features: erythroderma, well-confined healthy islands inside the inflammatory areas, keratoderma of the palms and soles, or visible follicular hyperkeratosis. Histological features were: perifollicular parakeratosis in 26 cases, hyper-keratosis in 27 cases; the number of granular layers varied greatly from case to case and within one specimen; acanthosis was eczematous in 20 cases and psoriasiform in 8 cases. Erythroderma in PRP in this study was self-healing, lasting 2–6 months with one exception, but PRP must as a whole be considered a mild, chronic disease lasting on average 5.7 years. Complete recovery occurred in only 8 patients and 5 of these had had an erythro-dermic onset. Serum vitamin A level and tolerance test were normal. No immunological aberrations could be found. The HLA phenotype frequencies in PRP patients did not differ significantly from those in a Finnish control population. The autoradiographic study of the epidermal cells showed an increase in the cell production in the epidermi