Ten long-term cyclosporine-treated patients with chronic uveitis underwent percutaneous renal biopsy in order to evaluate a) abnormalities of renal morphology and b) the nature of lymphocytic infiltrates by immunohistochemistry. Pretransplant renal biopsies from eleven cadaveric donors served as controls. Eight of the ten patients had lymphocytic infiltrates consisting predominantly of T lymphocytes with a CD4+/CD8+ ratio of 1.46, which is identical to that of peripheral blood in healthy donors. Evidence of immune activation as estimated by the presence of interleukin-2 receptors, transferrin receptors or by the expression of MHC class II antigens was not demonstrated in any of the patients. The severity of morphologic alterations did not correlate with any of the clinical or paraclinical data. Percutaneous renal biopsy should be performed within 18 months of treatment to identify patients susceptible to renal side effects of cyclosporine.