In the past it was not unusual for untreated, chronically obstructed/infected kidneys (pyonephrosis) to present as a loin mass/abscess.  It is now uncommon due to effective ureteric drainage with stents and liberal antibiotic use.  Occasionally it is now seen in individuals with poor access to medical care.  Treatment requires vigorous drainage of the abscess(es) and relief of ureteric obstruction.