Cyclosporin A nephrotox city was studied with the use of lithium and creatinine clearance tests in 18 patients with chronic intraocular inflammation (duration of treatment 3–48 months). In 11 patients lithium clearance and fractional excretion of lithium were significantly reduced (compared with pretreatment levels) within the first six months of treatment. There was no significant change in either serum creatinine or creatinine clearance within this period. In 14/18 patients there was a significant reduction in lithium clearance and fractional excretion of lithium during the treatment period. 7 patients whose therapy was stopped because of continuing nephrotoxicity despite dose reduction, demonstrated some reversibility of renal function on cessation of cyclosporin A. We propose a protocol for the assessment of renal function in these patients so that with dose modulation the changes in these parameters can be minimised, reducing the risk of renal impairment, whilst maximising immunosuppressive treatment.