Over 95% of UTIs are caused by a single bacterial species, and 90% of these are E. coli. Other Enterobacteriaceae, Pseudomonas, and Gram-positive bacteria become increas-ingly frequent with chronic, complicated, and hospitalized patients. Of the Gram-positive bacteria enterococci are the most important. Staphylococcus saprophyticus, a coagulase-negative staphylococcus, is now recognized as the etiology in a significant minority of symptomatic infections in young, sexually active women. Yeasts, particularly species of Candida, may be isolated from catheterized patients receiving antibacterial therapy andfrom diabetic individuals, but they seldom produce symptomatic disease.