OBJECTIVES: We studied the false-positive rate of bag urine cultures in diagnosing bacteriuria in infants and examined the factors responsible for contamination. METHODS: One hundred asymptomatic patients with previous urinary tract infections (UTI; age range 5-23 months; sex ratio M : F 73:27) were screened by bag urine cultures and confirmed by suprapubic aspiration or catheterization. Those producing contaminated and clean samples were interviewed with a standard questionnaire. RESULTS: Sixty patients had insignificant growth and 40 had doubtful or mixed growth. After further interview and instructions, the latter group produced a second bag urine sample with negative results in 23 and positive results in 17. Among the latter, UTI was confirmed in five and excluded in 12 cases. The false-positive rate of first urine collection was 36.8%. Comparing patients with contaminated urine to those with negative urine samples, we found the former were associated with waiting for more than one void to complete urine collection, and with uncircumcised boys. Other factors did not reach statistical significance. Proper counselling and repeating a second culture reduced the overall false-positive rate to 12.6%. CONCLUSION: In our population, the contamination rate of one bag urine culture was 36.8%, which is unacceptable and alternative methods need to be sought. Contamination was associated with improper collection procedures and with uncircumcised boys. Proper instructions and doing a second urine culture reduced the overall rate to 12.6%, but at the expense of delaying diagnosis and treatment.