Studies of remnant kidneys from dogs document the oc-currence of histologic lesions. These lesions include mes-angial cell proliferation, mesangial matrix accumulation,periglomerular fibrosis, tubulointerstitial infiltration withmononuclear cells, interstitial fibrosis, and mineral deposi-tion. These lesions are the same as those encountered inkidneys of dogs with the naturally occurring renal diseasecommonly described as chronic interstitial nephritis.Although empirical examination suggests that patholo-gists are unable to separate microscopic sections of remnantkidneys from kidneys of dogs with naturally occurring tu-bulointerstitial diseases, controlled comparison studies havenot been reported. In studies of dogs with remnant kidneys,initial and terminal kidney samples have been analyzed butserial samples from the same dog have never been reported.Thus, although histologic examination is considered a moresensitive indicator of self-progression than function tests,its usefulness in documenting the rate of progression is un-proven in this model.