ABSTRACTTopical application under occlusion of 0.1% Vitamin A Acid (VAA) in polyethylene glycol to the non‐involved skin of psoriatics and to the skin of normal controls resulted in keratolysis, acanthosis, glycogen accumulation, and increased enzyme activity, though the changes are more marked in the former. Those changes occurred during the first week of application and did not progress further, in fact continued application resulted in a biochemical return toward normal.A Koebner phenomenon was not seen and the test is therefore not useful in bringing out latent psoriasi