Interim results of a randomized, controlled, dose-finding study conducted in 24 dermatology centers on 217 patients with severe chronic plaque psoriasis (entry psoriasis area and severity index of at least 15) are presented. Patients were first treated with cyclosporine either 1.25 or 2.5 mg/kg/day (Sandimmune); in case of inadequate response the dosage was increased to a maximum of 5 mg/kg/day. Cyclosporine was given for 12 to 36 weeks. Treatment was classified as "successful" if the psoriasis area and severity index was reduced to 25% or less of the baseline value or below 8. At the end of the treatment period 18% of patients had improved their psoriasis area and severity index "successfully" with the initial dose of 1.25 mg/kg/day. "Successful" improvement with the initial dose of 2.5 mg/kg/day was achieved in 56% of the cases. In the 1.25 mg group, 44 patients had to increase their dose to 2.5 mg/kg/day and 31 patients to 5 mg/kg/day. In 29 patients whose dosages were started at 2.5 mg/kg/day the dosage was subsequently increased to 5 mg/kg/day. Although 1.25 mg/kg/day proved to be effective in some cases, 2.5 mg/kg/day of cyclosporine is the optimal starting dose. Adverse events were observed in 10% of the patients taking 1.25 mg/kg/day, in 20% taking 2.5 mg/kg/day, and in 32% taking 5 mg/kg/day. The most common were gastrointestinal complaints, followed by the common cold and other viral infections. An increase of serum creatinine level above 130 mumols/L occurred in five patients who could continue therapy after reducing the dose.(ABSTRACT TRUNCATED AT 250 WORDS)