Patients who were cigarette smokers suffering exacerbations of chronic bronchitis were examined in eight outpatient clinics in five regions of Italy, three from the South (Campania, 82 patients; Sicily, 82 patients; and Puglia, 29 patients) and two from North (Lombardy, 33 patients; and Liguria, 50 patients). Haemophilus influenzae was the most frequently isolated bacterium in the patients' sputum (in 30% of the total group), followed by Streptococcus pneumoniae (in 20%), Staphylococcus aureus (in 25%), and Branhamella catarrhalis (in 7%). H. influenzae was the most common bacterium in the South (in 37%) and S aureus in the North (in 13%). Smoking index scores (number of cigarettes smoked daily x years of smoking) were 827 in patients in whom H influenzae was isolated; 691 in patients with S aureus; 599 in patients with S pneumoniae; 542 in patients with B catarrhalis; and 446 in patients in whom no isolates were found. Pulmonary function was most severely decreased in patients positive for H influenzae and S aureus. The results indicate an association between heavy cigarette smoking and lower respiratory tract infections that is influenced by regional differences.