Retroperitoneal abscess may not be diagnosed timely occasionally. Here we report a patient who had nosocomial urinary tract infection related retroperitoneal abscess and subsequently developed a secondary internal fistula to the adjacent descending colon. Nosocomial UTI related retroperitoneal abscess can be a source of occult infection that responses poorly to antibiotic treatment. Retroperitoneal abscess can be further complicated with internal colonic fistula. Repeated fistulography and culture may be necessary to identify the extensiveness of the abscess cavity and the development of resistant strains. The presence of multiple drug resistance pathogens can perplex the treatment course and repeated drainage will be required.