Renal scarring is considered a criterion of reflux nephropathy and the end stage of pyelonephritis. Prednisolone, a strong anti-inflammatory drug, at doses of 1 or 2 mglkg prevented renal scarring in rats following infection withSerratia marcescens. Four or 8 mglkg of prednisolone, however, did not inhibit renal scar formation. In a time course experiment, renal scarring was prevented when 4-day treatment with prednisolone was initiated 2, 5, or 13 days after infection. These results show that prednisolone is effective in preventing such scarring and suggest the clinical use of this drug for preventing renal scar formation after pyelonephritis and reflux nephropathy.