This study describes the prevalence and degree of chronic cyclosporine-associated nephropathy and its risk factors. For this purpose we reviewed all available renal histology specimens in 169 bone marrow transplant recipients treated during an eight year period with cyclosporine for prevention of graft-versus-host-disease, and determined their pattern and degree of histomorphological changes. A total of 51 specimens obtained from 49 patients by biopsy (n = 12) or autopsy (n = 39) was evaluated. The pattern of histomorphological changes was compared with diagnosis, age, sex, and potential risk factors--such as cyclosporine dose, levels, duration of therapy, changes in serum creatinine and onset of hypertension. Morphological lesions of chronic cyclosporine-associated nephropathy were found in 67% of the specimens. They were more frequent and more severe with increasing duration of cyclosporine therapy, in patients with a higher increase in serum creatinine during the first 3 months and in patients given total-body irradiation for conditioning. These latter findings suggest that additional damage sensitizes the kidney to irreversible toxic effects of cyclosporine.