Background: Children admitted to the hospital with urinary tract infections (UTIs) receive empirical antibiotic therapy. There is limited information on bacterial resistance to commonly prescribed intravenous antibiotics or on the risk factors for increased resistance in these patients. Objectives: To determine the antibiotic resistance pattern in children admitted to the hospital with UTIs, and to determine if history of UTI, antibiotic prophylaxis, or vesicoureteral reflux increases the risk of resistant organisms. Design/Methods: We reviewed all of the cases of UTI in children up to 18 years of age who were admitted during a 5-year period to Children's Hospital of Wisconsin, Milwaukee. We recorded age, sex, culture and sensitivity results, imaging that was performed, and past medical history. Results: We identified 361 patients with UTIs. Escherichia coli caused 87% of the infections, although E coli was significantly less common in children receiving prophylactic antibiotics (58%; P<.001) or in children with a history of UTI (74%; P<.001). Resistance to cefotaxime sodium was 3% in the patients not receiving antibiotic prophylaxis, but was 27% in the children receiving prophylactic antibiotics (relative risk, 9.9; 95% confidence interval, 4.0-24.5; P<.001). Resistance to aminoglycoside antibiotics was 1% in the children not receiving prophylaxis and 5% in the children receiving prophylactic antibiotics. Conclusions: Children who are receiving prophylactic antibiotics and are admitted to the hospital for a UTI are often infected with an organism that is resistant to third-generation cephalosporins. These children are more appropriately treated with an aminoglycoside antibiotic.