To evaluate abnormalities of renal morphology associated with long-term cyclosporine therapy, we obtained percutaneous renal biopsy specimens from 17 patients who had been treated for autoimmune uveitis with cyclosporine for an average of two years. For comparison, we also analyzed renal biopsy specimens from patients with idiopathic hematuria who had not received cyclosporine. The atrophic and sclerosing glomerular and tubulointerstitial lesions in the specimens were assessed to derive a chronicity index. The specimens from the patients treated with cyclosporine showed a significantly higher chronicity index than those from the controls (P less than 0.00005). All patients who had received cyclosporine had interstitial fibrosis, tubular atrophy, or both. A good functional predictor of the abnormalities in renal morphology was the length of time that a patient's serum creatinine had exceeded the base-line value by more than 50 percent. Chronic pathologic alterations were seen even in patients who had normal renal function at the time of the biopsy. In conjunction with sequential measurements of renal function, renal biopsy may have an important role in defining the type and degree of nephrotoxicity that can develop in patients receiving long-term cyclosporine therapy for autoimmune disease.