Pyelonephritis is an uncommon cause of renal failure in the horse. The horses with the highest risk for urinary tract infections and pyelonephritis are multiparous mares with a history of dystocia, bladder paralysis, urinary incontinence, or urolithiasis. Bacteria cultured from urinary tract infections and pyelonephritis are typically coliforms or normal skin flora. Successful management of renal failure is largely dependent on early diagnosis. The case presented is of a nine year old, Quarter Horse brood mare with a typical history for a subacute pyelonephritis that was complicated by a chronic hydronephrosis in the contralateral kidney.