Ten patients with endogenous uveitis were in clinical remission attributable to treatment with cyclosporine and prednisone. After the cyclosporine dose was reduced by two thirds, these patients were randomly assigned to treatment with or without ketoconazole, a potent inhibitor of cytochrome P-450, in a double-masked placebo-controlled study. The dose was reduced over three days. During a three-month follow-up, no patients treated with ketoconazole had a relapse of uveitis, while four of six (66%) control subjects had a flare-up. Toxicity in the ketoconazole-treated group was limited to a transient decrease in glomerular filtration rate (20% from baseline) at one month in two of six (33%) patients. Renal function was stabilized by further reduction of the cyclosporine dose.