In order to determine the etiological agents and the rate of resistance to various antibiotics, 209 consecutive gram-negative bacteria isolated from children admitted to Hacettepe University Children's Hospital with urinary tract infections were investigated over a three-month period. Of these, 46 (22%) were nosocomial isolates. The most frequently isolated organism was E.coli (n: 141) followed by Klebsiella spp. (39), Proteus spp. (19), Pseudomonas spp. (8) and Enterobacter spp. (2). In vitro susceptibilities were evaluated by microbroth dilution method, following NCCLS guidelines. Overall, 75 percent of the isolates were resistant to ampicillin, 52 percent were resistant to TMP/SMX and 25 percent to cefuroxime. Amikacin was the most active aminoglycoside; 93 percent of the isolates were susceptible to this agent, while resistance to gentamicin was 21 percent. Resistance to ceftazidime and ceftriaxone was 12 percent and 19 percent, respectively. Overall, resistance to imipenem was one percent and to ciprofloxacin three percent. These in vitro results should be taken into account before initiating empirical therapy; broad spectrum antibiotics should not be used if the isolate is susceptible to the older drugs in order to prevent the increase in resistance.