The objective of this randomized, double-blind study was to compare the clinical efficacy of levofloxacin at two different dosages with that of cefuroxime axetil in patients with acute purulent exacerbations of chronic bronchitis and, in particular, to assess the impact of the susceptibility to levofloxacin on the clinical findings. In total, 124 evaluable patients were treated for 7 days with oral levofloxacin 250 mg or 500 mg od, or cefuroxime axetil 250 mg bd. Sputum cultures were monitored pre-treatment, and at 1 and 7 days after the end of treatment. The susceptibility of Streptococcus pneumoniae isolates was tested by agar dilution in Columbia blood agar and by disc diffusion, but all other isolates were tested solely by the disc diffusion method. A greater number of infections were eradicated by levofloxacin than by cefuroxime axetil: infections were eradicated in 68% of patients receiving the 500 mg dosage and in 63% of those taking 250 mg levofloxacin, whereas the eradication rate with the comparator drug was much lower (48%). Against all pre-treatment S. pneumoniae isolates (n = 39), the MICs of levofloxacin were between 0.25 and 2 mg/L (geometric mean 0.95 mg/L), similar to those of the post-treatment strains (n = 32; mean 1.11 mg/L). All except one of the S. pneumoniae isolates were susceptible to penicillin G (MIC < or = 0.06 mg/L), and the remaining isolate was inhibited by 0.5 mg/L of penicillin G, but was fully susceptible to levofloxacin. Some pretreatment strains of Pseudomonas aeruginosa were resistant to levofloxacin, but many more resistant strains were encountered afterwards. All strains of Moraxella catarrhalis and Haemophilus influenzae were highly susceptible to levofloxacin in the disc diffusion tests. All the antimicrobial agents used in the study were well tolerated: only two patients discontinued treatment because of adverse drug effects. The results of this study indicated that, although there were some failures in patients with S. pneumoniae and P. aeruginosa infections, resistance to levofloxacin did not emerge rapidly among strains of S. pneumoniae during therapy with levofloxacin, and that natural resistance among pneumococci, H. influenzae and M. catarrhalis was rare.