Like in any infection, the choice of antibacterials in pulmonary infections of known bacterial etiology is simple. When etiology is not known, the choice must rest upon knowledge of the epidemiology of lower respiratory infections and the antibacterial spectrum of the antibiotics in question. The epidemiology of community-acquired lower respiratory infections is not too well studied. However, some studies indicate that approximately 50% of lower respiratory infections are caused by bacteria among which Streptococcus pneumoniae prevails, followed by Haemophilus influenzae. Streptococci, Branhamella catarrhalis and other Neisseria species, staphylococci and Enterobacteriaceae account for less than 10% each. The prevalence of Legionella pneumophila is unknown, but it is of limited significance. Mycoplasma pneumoniae varies in prevalence according to time and geographic area. In acute exacerbations of chronic bronchitis, the epidemiology is similar, except that H. influenzae is more commonly found than pneumococci. The traditional strong position of penicillin in the blind, primary treatment of community-acquired lower respiratory infections is challenged by the increasing frequency of penicillin-resistant H. influenzae and the discovery of new agents not sensitive to penicillins. The same can be said for the more recently introduced primary treatment with erythromycin. However, most community-acquired infections in the lower respiratory tract respond to penicillin; tetracycline or erythromycin may be used for treatment when the clinical response is unsatisfactory. In patients who are known or suspected to have compromised host defense, beta-lactams such as ureido-penicillins and the new cephalosporins should be used as primary therapy. In hospital-acquired lower respiratory tract infections, the etiological diagnosis is more likely to be made.(ABSTRACT TRUNCATED AT 250 WORDS)