We studied the urinary IgG and IgA antibody in elderly subjects without bacteriuria, with asymptomatic bacteriuria and symptomatic urinary infection. Elderly individuals with asymptomatic bacteriuria had significantly elevated antibody levels relative to controls. When followed up to 12 months with persistent bacteriuria, antibody levels tended to persist but were variable. In elderly women with elevated urinary antibody titers, site of infection tended to be localized to the kidney, but antibody levels within the control range occurred as frequently as elevated antibody levels for renal infection. Subjects with invasive urinary infection had significantly elevated antibody levels at onset of symptoms, with levels increased or decreased when repeated 2–4 weeks later. Further studies to clarify the clinical significance of these observations are necessar