The increased incidence of infection and malignancy in elderly individuals has prompted many studies that demonstrate that the aging immune system is impaired. Most of these studies have focused on the impairment of acquired immunity provided by lymphocytes. While defects in acquired humoral and T-cell-mediated immunity may exist, increased susceptibility to infection may result from defects in the constitutive functioning of macrophages and granulocytes. Recognition of the potential importance of defects in constitutive immunity in the elderly may provide new opportunities for therapeutic and prophylactic intervention in this population.