In studies primarily designed to evaluate the effectiveness of chitosan as a treatment for cyclophosphamide-induced hemorrhagic cystitis in the rat, renal papillary necrosis and pyelonephritis were observed. Cyclophosphamide alone produced relatively mild renal changes. The combination of cyclophosphamide and intravesical instillation of acetic acid induced renal papillary necrosis (38 to 83% incidence) along with pyelonephritis, hydroureter and hydronephrosis. Chitosan, instilled in place of acetic acid, partially inhibited the induction of renal papillary necrosis. It appears that the presence of vesico-ureteral reflux with or without associated hydroureter and hydronephrosis is a prerequisite for cyclophosphamide-induced renal damage.