Bacterial growth in the urinary tract is usually prevented by host factors including bacterial eradication by urinary and mucus flow, urothelial bactericidal activity, urinary secretory IgA, and blood group antigens in secretions which interfere with bacterial adherence. Bacterial eradication from the urinary tract is partially dependent on urine flow and voiding frequency. Therefore, it seems logical to postulate a connection between fluid intake and the risk of urinary tract infections (UTIs). However, experimental and clinical data on this subject are conflicting. Experimental studies concerning the effect of water intake on susceptibility and course of UTIs were predominantly performed in the 60s and 70s. Despite many open questions, there has been no continuous research in this field. Only few clinical studies producing contradictory results are available on the influence of fluid intake concerning the risk of UTI. One explanation for the inconsistency between the data might be the uncertainty about the exact amounts of fluid intake, which was mostly recorded in questionnaires. So far, there is no definitive evidence that the susceptibility for UTI is dependent on fluid intake. Nevertheless, adequate hydration is important and may improve the results of antimicrobial therapy in UTI. Results of experimental and clinical studies concerning urinary hydrodynamics are the basis for advice given by expert committees to patients with UTI to drink large volumes of fluid, void frequently, and completely empty the bladder. The combination of the behaviourally determined aspects of host defence and not simply increasing fluid intake is important in therapy and prophylaxis of UTI.