Because of difficulties in accurately determining an etiologic diagnosis, the ideal treatment for lower respiratory tract infections remains questionable. Suggested regimens are made on the basis of clinical and epidemiological data. However, the single most common pathogen responsible for pneumonia remains Streptococcus pneumoniae. Atypical pneumonia in younger patients is best treated with macrolides. Older patients without debility or immunodepression are best treated with amoxycillin-ampicillin, second generation cephalosporins or cotrimoxazole, on the basis of local susceptibility patterns of microorganisms. In the treatment of acute bacterial bronchitis in chronic bronchial disease, most antimicrobial agents with activity in vitro against Haemophilus influenzae and Streptococcus pneumoniae are clinically efficacious. Among new pathogens, the importance of Chlamydia pneumoniae is variable according to the studies, and Moraxella catarrhalis was considered almost exclusively responsible for purulent exacerbations of chronic bronchitis. Therapy for empiric treatment of nosocomial pneumonia must ensure coverage for aerobic Gram negative bacilli: the most frequently used includes a semisynthetic penicillin plus an aminoglycoside, but monotherapy with newer broad-spectrum antibiotics (imipenem, ceftazidime, ciprofloxacin, timentin, etc.) seems to be equivalent to combination regimens. The lung is the most common target organ for infectious complications in immunocompromised patients but the diagnostic methods employed in the traditional work-up of pneumonia are often of little or no use in this setting. By far the two most useful clues to management of pneumonia in the immunocompromised host are the underlying host defect and the radiographic pattern of the lung infiltrate.(ABSTRACT TRUNCATED AT 250 WORDS)