The effectiveness of cranberry in the treatment of urinary tract infection (UTI) has been associated with its polyphenol content, particularly proanthocyanidins (PACs) and the inhibition of adherence of Escherichia coli to the uroepithelium. This paper describes a controlled, double blind, clinical trial of children aged over one month with recurrent urinary tract infection. The study aims were to evaluate the safety and efficacy of cranberry syrup in children and to investigate the relationship between the excretion of phenolic acids in urine with the antiadherent activity of cranberry syrup. In the study population, cranberry syrup was found to be similar to trimethoprim, with a rate of UTI (reinfection) of 26% (95% CI 12-41). The administration of cranberry syrup was associated with high levels of hydroxycinnamic and hydroxybenzoic acids in urine; in both cases these molecules present activity in the biofilm inhibition or reduction of surface hydrophobicity of E. coli (Clinical Trials Registry ISRCTN16968287).