Urinary catheters and legs bags were simultaneously colonized by Escherichia coli and Proteus vulgaris using a model urinary drainage system. the system was continuously supplied with filter-sterilized artificial urine using a diurnal flow pattern. The extent of colonization was determined by assessment of both planktonic and biofilm formation over time. Contamination of the catheters resulted in rapid colonization of the whole system within a 24 h period. Contamination of the leg bags resulted in an ascending biofilm formation over a four-day period. Results indicated that infection risk could be minimized by changing the catheter and leg bags at least once a week. The design of the leg bags was not found to influence the rate or extent of biofilm formation.