Purpose We studied the clinical effects of the immunosuppressive agent FK506 in patients with noninfectious uveitis. Methods This study was designed as a multicenter open clinical trial in Japan. Sixteen patients with noninfectious uveitis who had visited the Uveitis Survey Clinic of the Yokohama City University Hospital were given FK506. Eight had Behçet's disease; five, Vogt-Koyanagi-Harada syndrome; one, sympathetic ophthalmia; one, retinal vasculitis; and one, sarcoidosis. In patients with Behçet's disease, ocular attack score before and after therapy was compared to judge clinical status. For the other diseases, the ocular inflammatory symptoms were observed after the initiation of FK506 treatment. All patients underwent blood and urine examinations, electrocardiography, and chest x-rays before and after FK506 treatment. Results Of the patients with Behçet's disease, five improved, one remained unchanged, one deteriorated, and the status of one could not be determined. Of the patients with Vogt- Koyanagi-Harada syndrome, four improved, and one remained unchanged. The patient with sympathetic ophthalmia improved, the patient with retinal vasculitis remained unchanged, and the status of the patient with sarcoidosis could not be determined. Major adverse effects were sensations of warmth, hypomagnesemia, renal dysfunction, glucose intolerance, nausea, vomiting, and disorders of the central nervous system. All adverse effects disappeared or improved when FK506 was stopped or when the dosage was decreased. Renal dysfunction and glucose intolerance appeared when the blood level of FK506 was high. Conclusions FK506 was effective in patients with uveitis, but it is important to monitor the occurrence of adverse effects.