The populations at risk for urinary tract infection include the newborn, particularly the premature, prepubertal girls, young boys, sexually active young women, elderly males, and elderly females. Risk factors that contribute to lower tract infection in women include sexual intercourse, diaphragm-spermicide use, and voiding behavior. Host factors, more than bacterial virulence, are probably the most important contributors to infection. The genetic factors that are important contributors are secretor status and P blood group phenotype. Which patients to culture, when to culture, and the number of organisms required to define infection have changed in the past decade. A concentration of 10(2) colony forming units/mL can cause an acute urinary tract infection in the healthy woman. The presence of leukocytes in the urine is of increasing diagnostic importance. Complicated urinary tract infections occur in neonates with such congenital anomalies of the urinary tract as urethral valves or in patients with neurologic disease resulting in urinary stasis. In older men or women, complicated urinary tract infections occur with obstruction, instrumentation, surgery, anatomic abnormalities, or stones. Single-dose therapy of uncomplicated urinary tract infection is useful in only a small subset of patients, specifically in patients less than 45 years of age who have short duration of symptoms. The majority of patients with uncomplicated infections should receive treatment for 3-5 days. Response to therapy and long-term cure rates in complicated urinary tract infection are related both to the type of underlying abnormality and to the species of the infecting organism. Complicated urinary tract infections should be treated for 7-14 days.