Urinary tract infection (UTI) is an infection anywhere in the urinary tract, most commonly due to bacteria. If infection involves the kidney, the UTI is termed acute pyelonephritis (APN). An estimated 10-30% of all patients with APN are hospitalised for treatment; in the US, the incidence of hospitalisation is 11.7 per 10,000 for women and 2.4 per 10,000 for men. Perhaps because of the generally good prognosis of APN when treated with current antibacterial therapies, there have been relatively few studies of patient management and therapeutic options for the disease, or of its epidemiology and risk factors. The most cost-effective outpatient management strategy (immediate discharge, observation followed by discharge, etc.) is currently unknown. Appropriate antimicrobial selection is clearly important, as treatment failures will increase the cost of care and result in additional morbidity for patients. The direct and indirect costs of APN are significant: an estimated 2.14 billion US dollars (year 2000 values). Cost estimates are most sensitive to hospitalisation rates, which are unknown in the US. Additional studies are needed to better define when in-hospital treatment is required. As the pathogens causing APN are increasingly becoming resistant to current therapies, not only are clinical trials in order to test the effectiveness of alternative therapies, but epidemiological studies to identify risk factors for infection with a resistant isolate and effective prevention strategies are required, especially among those with previous episodes of APN.