Patients with chronic obstructive pulmonary disease are generally subjected to multiple regimens of antimicrobial treatment. The development of high-level levofloxacin resistance (i.e., a minimum inhibitory concentration 18 mg/mL) in 8 patients whose previous pneumococcal isolates showed susceptibility is described. Molecular methods were used to characterize the strains and to study the sequential changes in fluoroquinolone targets.The prevalence of fluoroquinolone-resistant Streptococcus pneumoniae remains low in most parts of the world, but it seems clear that, as the use of fluoroquinolones increases, the possibility that resistant isolates will appear will also increase. The tetrameric enzymes DNA gyrase (encoded by the gyrA and gyrB genes) and DNA topoisomerase IV (encoded by the parC and parE genes) are intracellular fluoroquinolone targets. Most levofloxacin-resistant clinical isolates possess single mutations in a discrete region of the genes encoding the ParC and GyrA subunits of topoisomerase IV and DNA gyrase, termed the quinolone resistance-determining region (QRDR). Although there are many reports concerning the prevalence of fluoroquinolone resistance in S. pneumoniae, few reports describe the emergence of this resistance during treatment.S. pneumoniae is a frequent cause of acute bacterial exacerbation of chronic obstructive pulmonary disease (COPD). Patients with COPD are subjected to frequent courses of antimicrobial treatment because of acute bacterial exacerbations. Because previous antimicrobial use has been directly associated with the development of antimicrobial resistance in S. pneumoniae [1], it is not surprising that drug-resistant pneumococci are more prevalent in patients with COPD than they are in the general adult population [2]. This finding prompted us to conduct a surveillance study to analyze the changes in the antimicrobial susceptibility patterns of pneumococcal strains causing repetitive episodes of infection, focusing on fluoroquinolone resistance. The present study describes the development of levofloxacin resistance during treatment, the sequential genetic changes observed in the DNA gyrase and topoisomerase IV of successive isolates, and the persistence of colonization with the same strain over time in patients with COPD receiving fluoroquinolone therapy.From January 1999 through November 2004, we searched for patients who had at least 2 S. pneumoniae isolates that were obtained during episodes of acute exacerbation of COPD. Only patients whose isolates were of the same serotype and genotype and for whom the first isolates were levofloxacin susceptible and the subsequent isolates were levofloxacin resistant were included in the study. The design of the study was prospective and descriptive, because we did not influence treatment or the number of samples requested from the patients included in the study. Some patients had more samples than others, depending on the number of exacerbations of their respiratory process and on the criteria of the pulmonologist or general pract...