The incidence of skin cancer in patients with psoriasis seems to be low, despite repeated use of known carcinogens in treating the disorder. This may be due to a reduced capacity of psoriatic skin to metabolise precarcinogens because of impaired arylhydrocarbon hydroxylase (AHH) activity. If this hypothesis is correct, and impaired AHH activity in psoriatic skin is shared by other tissues, the incidence of cancers associated with environmental carcinogens may also be reduced in patients with psoriasis. Since the disease is probably genetically transmitted as a dominant trait, it may have persisted because it confers genetic advantage. This hypothesis, though speculative, provides a basis for further study.