In most episodes of urinary tract infection bacteria reach the bladder through the urethra and then may ascend to the kidneys through the ureters. Bacteria periodically enter the female urinary bladder from the urethra in small numbers. Whether infection ensues depends on the virulence and inoculum size of the microorganism and the adequacy of most defense mechanisms. Human urine is frequently inhibitory and sometimes bactericidal for the bacteria that cause urinary tract infection. Inhibition of bacterial growth coupled with voiding serves as a very effective antibacterial defense mechanism. In addition an intrinsic antibacterial defense mechanism has been demonstrated in the rat urinary bladder unrelated to urine flow. The medulla of the kidney is much more susceptible to infection than the cortex mainly related to its high osmolality. Certain abnormalities of the urinary tract interfere with host defense mechanisms and therefore predispose to urinary tract infection or make infection more difficut to eradicate once present. Some of these abnormalities are obstructive or neurological diseases of the urinary tract, vesicoureteral reflux, calculi, certain metabolic, hematologic, and vascular diseases, and pregnancy.