Arm I patients continue treatment for at least 2 months. At the end of 2 months of topical treatment, one biopsy is taken of the lesion treated with calcitriol, one biopsy of the lesion treated with petrolatum alone, and one biopsy of skin unaffected by psoriasis. Lesions are also photographed prior to therapy, 2-4 weeks during therapy for the first two months, then once every 1-3 months, and then once at end of treatment.