Nasopharyngeal specimens for culture of Chlamydia pneumoniae were obtained from patients participating in two pneumonia treatment studies: an open study of 400 mg moxifloxacin orally, qds for 10 days and a randomized, double-blind comparison of moxifloxacin, 400 mg orally, qds versus clarithromycin, 500 mg orally, bd, both for 10 days. C. pneumoniae was eradicated from the nasopharynx of seven of ten (70%) microbiologically evaluable patients who were treated with moxifloxacin and four of four who were treated with clarithromycin. Minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) of 21 isolates of C. pneumoniae from 18 patients obtained before and after therapy were performed against moxifloxacin and clarithromycin. The MIC(90)s and MBC(90)s for moxifloxacin and clarithromycin were 1 and 0.06 mg/l, respectively. The MICs and MBCs against moxifloxacin of six isolates from three persistently infected patients who were treated with the drug were the same at baseline and follow-up. The persistence of C. pneumoniae after treatment with moxifloxacin was probably not due to the emergence of resistance.