Background: Cyclosporine is very effective in the treatment of severe, recalcitrant psoriasis. The absorption of previous formulation of cyclosporine is affected by food intake and fat content, by the dispersion of the agent in the gastrointestinal tract and by the secretion of bile. As a result, the use of cyclosporine is limited by a degree of variability in cyclosporine bioavailability, both between patients and within an individual patient over time. Microemulsion formulation of cyclosporine has an improved bioavailability and linear dose response over a wide dosage range and more constant serum concentration. The purpose of this study was to evaluate the efficacy and safety of a low-dose microemulsion cyclosporine therapy for psoriasis. Methods : Open uncontrolled multicenter study was performed in 6 university hospitals in Korea. There were 74 total trial cases and among them 10 patients dropped out from the study. 64 patients with moderate to severe psoriasis were treated with an initial dose of 2.5 mg/kg per day of microemulsion cyclosporine for 18 weeks. The initial dose was main tained or increased to 4 mg/kg/day or 5 mg/kg/day according to the PASI score reduction rate at the 6th and 12th week. The PASI score was measured, and laboratory tests and observation of adverse effects were done at the 6th, 12th and 18th week. Results: The mean PAS! scores reduced statistically significantly from 24.4 to 4.3 after the cyclosporine treatment for 18 weeks. The reduction rate of PASI score of more than 67 % was achieved in 89.7% of patients at the 18th week. Changes in systolic and diastolic blood pressure were not statistically significant. Pruritus was significantly reduced but the nail involvement was not significantly improved by the cyclosporine treatment. The mean concentrations of serum creatinine and BUN were not significantly increased and only 3 patients showed increased serum creatinine lebel transiently. The increase of serum AST, ALT and total bilirubin was observed in 4 patients (6.3%), 13 patients (20.3%) and 10 patients (15.6%), respectively. These abnormalities were transient and none of these patients stopped the cyclosporine treatment. The clinical adverse effects reported during the study were gastrointestinal discomfort (10 cases, 15.6%), arthralgia (2 cases, 3.1%), hypertrichosis (2 cases, 3.1%), palpitation (1 case, 1.6%) and flu-like symptom (1 case, 1.6 %). The overall assessment of efficacy and tolerability of microemulsion cyclosporine treatment by investigators and patients were mostly very good or good. Conclusion : Generally, microemulsion cyclosporine treatment was well accepted and tolerated by the patients. The low-dose microemulsion cyclosporine treatment is an effective and safe therapeutic modality for the treatment of psoriasis.