Changes in renal physiology and function with aging--eg, morphologic changes, decreased renal blood flow, altered glomerular filtration rate, and blunted tubule response--put the elderly patient at higher risk for adverse effects of drug therapy and for undertreatment. A thorough understanding of the many age-related alterations in kidney function will lead to the selection of appropriate therapy for elderly patients and, thus, the prevention of common problems such as volume depletion, infection, hyponatremia, metabolic acidosis, and excessive dosing.