How common is psoriasis among the general population? One searches the dermatologic literature in vain for an answer to this question. Yet more people have psoriasis than any other disease of the skin. Eczema, dermatophytosis and acne vulgaris outrank it in frequency at a dermatologic clinic, but none of these dermatoses lasts a lifetime. I have tried to determine the incidence of psoriasis in a crude way from the meager statistics available. Psoriasis begins in about one-half the cases before the age of 20. The average age of patients with psoriasis at their first visit to a dermatologic clinic is, therefore, lower than that of persons with many dermatoses which appear in adult years, for example, herpes zoster and lupus erythematosus. Since there is no evidence that the life of a person suffering from psoriasis is shortened, it is not unreasonable to assume that his average expectation of life