Cyclosporin A (CYA) has recently attracted the attention of the dermatologists with regard to its use in the treatment of severe psoriasis. We report four cases of psoriasis resistant to conventional therapy. Three patients were affected by chronic plaque psoriasis and one by erythrodermic psoriasis. The initial dosage of CYA was 5 mg/kg/day per os, and was gradually reduced according to the course of the disease. The drug was administered for a period of 6 weeks. All patients showed a significant improvement of the clinical picture. No remarkable clinical side effects were observed. During the trial we took skin biopsies on which we conducted histological and immunohistochemical studies. The efficacy of CYA in the treatment of psoriasis demonstrates the important role of the cell mediated immunity in the pathogenesis of psoriasis.