PURPOSE: Since children under two years with suspected urinary tract infections (UTIs) cannot control urination, urine cultures in such children are usually performed via urine bags. This method is noninvasive but has a high contamination rate. We studied the contamination rate of bag urine culture in diagnosing UTIs in infants under two years and the factors responsible for contamination. METHODS: We examined patients under 2 years in whom urine culture through the urine bag method yielded over 105 colonies of a single pathogen. We defined UTIs by referring to the guidelines of The Korean Society of Pediatric Nephrology, 2005. We examined the factors responsible for contamination according to sex, duration of urine collection, and whether diarrhea took place with contamination rate. RESULTS: We examined 717 patients (412 males and 305 females). The contamination rate of one bag urine culture was 37.9%. Gender was not related to the contamination rate (P>0.05). Duration of urine collection showed an association with the contamination rate. The longer the duration of collecting urine, the higher was the contamination rate. Duration of urine collection was divided into three groups: first group, <2 hours; second group, 2-4 hours; and third group, > or =4 hours. The contamination rates were 30.0%, 42.2%, and 43.7% for the first, second, and third groups, respectively, with statistical significance (P=0 .001). Diarrhea at admission had no impact on the contamination rate (P>0.05). CONCLUSION: The contamination rate of urine culture in the examined patients was 37.9%. Gender and diarrhea symptoms were not responsible for contamination. In infants with a suspected UTIs, urine should be collected within 2 hours through the urine bag method. If urine collection takes over 2 hours, the urine bag should be resterilized and reattached to the patient.