To demonstrate the infection rates, colonization rates following the internal ureteral stent placement; the correlation of indwelling time with the infection, bacterial colonization and the value of urine culture to identify colonizing bacteria One hundred and forty-eight stents of 146 patients were evaluated for the bacteriuria and colonization after internal ureteral placement average 8.6 weeks according to the indication. All patients were ambulatory and were examined in an outpatient clinic. Urine culture and 1 cm of proximal and distal ends was taken for culture for bacterial evaluation. The rate of colonization is 33, 50 and 54% when indwelling time is less than 4 weeks, 4-6 weeks and more than 6 weeks, respectively. Urine culture can detect colonization in 69%. Colonization was not found if the indwelling time was less than 2 weeks. Escherichia coli, Enterobacter and Pseudomonas spp. were the most common colonized organisms. Colonization is common if the indwelling time is more than 2 weeks, urine culture can detect colonization in about two to three of the patients. Even if the culture is negative, prophylactic antibiotic that cover gram-negative and gram-positive organisms should be administrated when the patients require further procedures. © Springer-Verlag 2006.