OBJECTIVE To determine the relationship between obesity and febrile urinary tract infection (UTI) in young children. METHODS We reviewed all medical records of children aged <3 years who visited our institution for febrile UTIs and infant national health checkups (controls) between January 2008 and February 2012. All subjects were subcategorized into 3 groups of lean, overweight, and obese using weight-for-length measurements. The effect of obesity on UTI risk was evaluated and odds ratios were calculated. RESULTS We analyzed 465 patients with UTIs and 812 controls. The proportion of overweight and obese children was higher in patients with UTIs (22.8%) and acute pyelonephritis (APN; 31.1%) compared with those in the control (11.7%; P <.05). After adjusting for age and gender, the odds ratio (OR) of UTI in obese relative to lean children was 1.84 (95% confidence interval [CI], 1.11-3.05) and that of APN was 2.43 (95% CI, 1.27-4.62). The OR of APN in overweight relative to lean children was 1.96 (95% CI, 1.11-3.46). After adjusting for age, the OR of APN in obese relative to lean boys was 2.74 (95% CI, 1.11-6.77) and that in overweight to lean girls was 2.48 (95% CI, 1.05-5.83). Within patients with UTIs, compared with lean children, the obese showed a longer duration of fever and a higher frequency of APN and the overweight had a higher incidence of hydronephrosis (P <.05). CONCLUSION Obesity may be associated with higher odds of a febrile UTI and APN in young children.