OBJECTIVE:Few studies have been carried out to determine the prevalence of microorganisms causing exacerbations of chronic bronchitis in the community setting. The aim of the present study was to determine the bacterial etiology of chronic bronchitis exacerbations in patients not requiring hospitalization. PATIENTS AND METHODS:This observational, cross-sectional, multicenter study was carried out at the primary care level during 2 weeks (in November 2001 and January 2002). All laboratory work was carried out at a single center. We studied 1,947 patients with mild-moderate exacerbations treated by 650 primary care physicians. All the sputum samples received for centralized processing were subject to Gram staining, microscopic examination, and bacterial culture. RESULTS:Out of 1,537 cultures of sputum samples collected, 498 had good cell quality for microscopic examination (32.4%). Of the 498 good quality samples analyzed, 246 (49.4%) were positive and 468 isolates were obtained. The most commonly isolated germ was Streptococcus pneumoniae (163 cases, 34.8%), followed by Moraxella catarrhalis (112, 23.9%), and Haemophilus influenzae (59, 12.6%). In 1.2% of the S. pneumoniae isolates resistance was found to amoxicillin; resistance to macrolides was found in 34.3%. The antibiotics most commonly prescribed, however, were macrolides (38.3% of the prescriptions). CONCLUSIONS:S. pneumoniae was the microorganism most frequently isolated in cases of chronic bronchitis exacerbation treatable in this outpatient setting.