Fleroxacin, administered intravenously at a dosage of 400 mg once a day, was compared with ceftazidime, 0.5-2 g three times daily or 1-2 g twice daily, administered for 4-21 days, for treatment of nonpneumococcal lower respiratory tract infections. A total of 319 patients were enrolled and randomized to receive treatment with fleroxacin or ceftazidime in a 2:1 ratio. Of those enrolled, 68 fleroxacin- and 49 ceftazidime-treated patients were included in the efficacy analysis. The most common diagnoses were pneumonia or pneumonitis (47% of the fleroxacin group and 57% of the ceftazidime group) and exacerbation of chronic bronchitis (38% and 33%, respectively). In the fleroxacin group, 59 (88%) of 67 patients were bacteriologic cures, and in the ceftazidime group, 40 (90%) of 49 were bacteriologic cures. It could be concluded with 95% confidence that the bacteriologic outcomes, by infection, for the two groups were equivalent (fleroxacin, 88%; ceftazidime, 90%). The rates of clinical cure were 59 (88%) of 67 for the fleroxacin group and 40 (82%) of 49 in the ceftazidime group, but since the 95% confidence limit around the between-group difference was greater than the stipulated +/- 15%, it could not be concluded that the outcomes were equivalent. The percentage of patients who experienced adverse clinical or laboratory events was similar in the two treatment groups (12% and 13%). The bacteriologic outcomes, by infection, were equivalent for the two treatment groups. Protocol requirements permitting a determination of equivalence of the outcomes for clinical cure were not met, although the rates were similar.