Trovafloxacin is a new fluorinated naphthyridone having expanded activity against Gram-positive and anaerobic pathogens compared with ciprofloxacin or levofloxacin or ofloxacin. A multicenter in vitro trial (201 sites) was initiated in late 1997 to study the comparative activity of trovafloxacin against Streptococcus pneumoniae strains during the recently completed respiratory disease season. Each laboratory was asked to test 20 to 30 recent isolates (3049 strains) by the Etest (AB BIODISK, Solna, Sweden) method with observed phenotypes with elevated trovafloxacin results (MIC, > 1 microgram/mL) confirmed by the monitor laboratory (University of Iowa College of Medicine). Approximately one-third (34.0%) of isolates were penicillin nonsusceptible (12.8% high-level resistance at > or = 2 micrograms/mL). Also 20.4% and 4.5% of strains were resistant to macrolides (erythromycin) and ceftriaxone, respectively. The macrolide resistance rate was lowered to 16.8% when the adverse effect of CO2 incubation was considered. Only 0.3% of S. pneumoniae were vancomycin-nonsusceptible using the current National Committee for Clinical Laboratory Standards breakpoint (< or = 1 microgram/mL) with nearly all results were at 1.5 micrograms/mL. Trovafloxacin (MIC50 and MIC90, 0.094 and 0.19 microgram/mL, respectively) was eight fold more potent than levofloxacin (MIC90, 0.75 and 1.5 micrograms/mL), and fewer strains (0.10%) were discovered with high-level resistance (MIC, > or = 8 micrograms/mL). The four resistant isolates from different states had alterations in both par C and gyr A. Trovafloxacin had the best potency observed against contemporary pneumococcal isolates, and has a spectrum (> 99.8% susceptible) for an orally administered agent that was comparable to the tested parenteral glycopeptide, vancomycin (> 99.7%). Blood and spinal-fluid culture isolates were generally more susceptible to penicillin (74.4 to 75.6%), other beta-lactams, and erythromycin (84.4%); throat and sputum isolates were significantly more resistant (p < 0.01). Increases in resistance among S. pneumoniae strains to beta-lactams and erythromycin were documented in all geographic regions monitored, other resistances also continue to evolve, and high-level fluoroquinolone resistance remains very rare.