Respiratory tract infections are a leading cause of morbidity and mortality in the elderly. Many factors, such as malnutrition and the presence of structural lung disease, increase the risk of respiratory infection in older individuals. Aging is also accompanied by a gradual decline in many aspects of immune function, and waning immunity is thought to be an important risk factor for pneumonia in the elderly. Although a generalized decline in both the cell-mediated and humoral aspects of acquired immunity have been described in otherwise normal elderly populations, relatively little is known about the effect of age on compartmentalized pulmonary immune surveillance and immune responses to a challenge with a respiratory pathogen. Changes in immune cell profiles and acellular components of bronchoalveolar secretions have been detected by bronchoalveolar lavage, but the impact of these changes on host defense against respiratory infections is unknown. An improved understanding of the age-associated changes in pulmonary host defense mechanisms and how these might be manipulated to reduce the susceptibility of the elderly to respiratory tract infections may reduce the possibility of severe debilitation and death and the considerable health care burden posed by the increased incidence of pneumonia in this at-risk population.