We performed a clinical open trial to evaluate the efficacy and the adverse side effects of a single therapy with FK506 in refractory uveitis as a multicenter study in Japan. Fifty-three patients (41 patients with Bechçet's disease, five with Vogt-Koyanagi-Harada disease, four with idiopathic retinal vasculitis, and three with other forms of uveitis) were enrolled in the study. FK506 was given orally for 12 weeks. Treatment with FK506 exhibited therapeutic effects in a dosage-dependent manner: the effectiveness was 38% in patients treated with an initial dosage of 0.05 mg/kg of body weight per day, 60% with 0.10 mg/kg of body weight per day, 83% with 0.15 mg/kg of body weight per day, and 79% with 0.20 mg/kg of body weight per day. Overall efficacy with dosage adjustment when needed was 76.5% at the conclusion of the study at the end of the 12th week. The FK506 therapy induced a variety of adverse side effects, the incidence of which depended on the dosage. The major side effects were renal impairment (28.3%, 15 of 53 patients), neurologic symptoms (20.8%, 11 of 53 patients), gastrointestinal symptoms (18.9%, ten of 53 patients), and hyperglycemia (13.2%, seven of 53 patients). The trough level of FK506 in the whole blood correlated with both the efficacy of the therapy and with the incidence of adverse effects. It is recommended to maintain the trough level between 15 and 25 ng/ml. On the basis of these results, a daily dosage of 0.10 to 0.15 mg/kg of body weight per day was suggested as an appropriate therapeutic dosage for refractory uveitis.