Urinary tract infection (UTI) is one of the most common bacterial infections in children. The symptoms of UTI in young children are nonspecific, therefore urine should be examined whenever UTI cannot be ruled out. In clinical settings, however, collecting urine from children who are not toilet trained is sometimes difficult, presenting a challenge in UTI management. Here, we developed a "diaper UTI test", which enables the quick detection of pyuria in ordinary diapers, and investigated its sensitivity and specificity in a clinical study. The diaper UTI test is based on a leukocyte esterase reaction. Reagent was prepared in liquid form so that it can be absorbed by disposable diapers, where it will produce a violet color in the presence of pyuria. For the clinical study, we enrolled children younger than 3 years with potential UTI who underwent bladder catheterization for urine culture and urinalysis. Of the 65 children included, 21 were diagnosed with UTI. The sensitivity and specificity of the diaper UTI test were 90.5% (95% CI 69.6-98.8) and 93.2% (95% CI 81.3-98.6), respectively. Because of its convenience and good sensitivity, the diaper UTI test may be useful in the screening of pediatric UTI.