The pathogenesis of psoriasis is not fully understood, but two mechanisms seem important, i.e. epidermal proliferation and an inflammatory process. Among these, the cytokines production and the expression of adhesion molecules, are the most important. To make an exact diagnosis of psoriasis it is essential to have the case history and the clinical findings. In difficult cases it might be necessary to make a biopsy. If the patient has not been treated, the histopathology is very characteristic. There are two main possibilities for treatment: either local therapy or internal therapy. Local treatment usually starts with the treatment of keratotic plaques, mostly by baths (salt or sulphur), followed by ointments or oils containing salicylic acid 5% or 10%. After a couple of days a specific local treatment is started. In the clinic or in a day care center one might use the classical therapeutic modalities such as dyes (Castellani) or tar or dithranol preparations. The inconveniences of these modalities are that they either color the body or the textiles or that they smell. These preparations, especially in combination with UV light, are very effective and useful. On an outpatient basis corticosteroid preparations are mostly used, but in certain skin areas they might provoke skin atrophy. With the development of the vitamin-D derivative calcipotriol, further progress has been achieved. This preparation is not colored, does not smell and is not followed by atrophy. The efficacy of calcipotriol is similar to potent corticosteroids. Calcipotriol can very well be combined with UV therapy. UV therapy modalities consist in solar light therapy (sea shore or mountains), UVB phototherapy, SUP therapy, or PUVA therapy.(ABSTRACT TRUNCATED AT 250 WORDS)