Urinary tract infections (UTIs) are more common among women than men, although the prevalence in elderly men and women is similar. Most of the research on UTI has focused on young, sexually active women who are at high risk for developing an infection. The predominant UTI risk factors in young women are sexual intercourse and the use of spermicidal contraceptives. Other important UTI risk determinants in selected age groups include anatomic and physiologic factors, such as obstructing lesions and estrogen deficiency; genetic factors, such as blood group secretor status; antibiotic exposure; functional status; and possibly receptive anal intercourse and HIV infection.