Tobacco smoke can alter both the structural and the immunologic defenses of the lungs against infection. The gases from smoke can also inhibit repair of the damages they create. Smokers have been found to have increased bacterial adherence to the respiratory tract, decreased IgA and IgG, and a decrease in vital capacity two to three times greater than that of nonsmokers. These and other respiratory tract alterations put smokers at an added disadvantage when acute lower respiratory tract infections strike. A history of smoking influences the diagnosis of respiratory tract infections because smokers are more prone than nonsmokers to infection by certain organisms. The major causes and characteristics of lower respiratory tract infections, methods of outpatient diagnosis, and the pros and cons of various modes of therapy are discussed.