Psoriasis is a disorder of abnormal keratinocyte proliferation mediated by T lymphocytes. Initiation of psoriasis is associated with an influx and activation of CD4 T lymphocytes into the dermis and epidermis, and resolution with an influx and activation of CD8 T lymphocytes within the epidermis. As a result of these observations it was postulated that cyclosporin (CsA) should prove to be an effective treatment for psoriasis. A total of 82 patients with psoriasis have been treated with CsA. Thirty patients with moderate disease were treated for periods up to 12 weeks, and comprised a short-term study. Two groups have been treated with long-term CsA. A group of 18 patients have been treated for greater than 1 year but less than 3 years, (mean 2.1) and a group of 10 patients for 4.0-5.5 (mean 5) years. A total of 15% of patients can be maintained on less than or equal to 2 mg/kg/day; 55% on 3 mg/kg/day, 80% on 4 mg/kg/day, and 92% on 5 mg/kg/day. Hypertension occurred in 17% of patients in the short-term study, 29% in the 2.1-year group and 44% in 5-year group. Blood pressure returned to normal in all hypertensive patients when CsA was discontinued for 1 month. Nephrotoxicity was assessed by serum creatinine levels and glomerular filtration rate (GFR) and renal biopsy in the 5-year group. In the short-term group there was no significant rise in the serum creatinine.(ABSTRACT TRUNCATED AT 250 WORDS)