To compare the efficacies of 10-day regimens of grepafloxacin (GFX) (Raxar or Vaxar; Glaxo Wellcome; Greenford, UK), 600 qd, and clarithromycin (CLA) (Klacid, Biaxin, or Klaracid; Abbott Laboratories; Chicago, IL), 500 mg bid, in patients with community-acquired pneumonia (CAP), on the basis of clinical response, including radiographic evidence, and bacteriologic efficacy. Phase IIIb, double-blind, double-dummy, randomized, prospective, parallel-group, comparative study conducted at 58 centers in 11 countries. Adult patients with signs and symptoms of CAP that was confirmed by radiographic evidence and who did not require parenteral therapy were included in the study. Patients were assessed before treatment, during treatment, after treatment, and at follow-up (28 to 35 days after treatment completion). Clinical response was evaluated. Blood and sputum samples were cultured for bacterial pathogens, and serology testing was performed to detect atypical pneumonia. A total of 504 patients were enrolled into the trial: 251 were randomly assigned to receive GFX and 253 to receive CLA. In patients able to be clinically evaluated, clinical success rates at follow-up were 147 of 163 patients (90%) in the GFX group and 148 of 167 patients (89%) in the CLA group (95% confidence interval, -6% to 9%). Pretreatment pathogens were confirmed in 131 of 504 patients (26%), either by culture or serology testing, the primary pathogens being Streptococcus pneumoniae (22%), Haemophilus influenzae (17%), Mycoplasma pneumoniae (25%), and Chlamydia pneumoniae (11%). For patients able to be evaluated who had typical pathogens, bacteriologic success was achieved in 92% of the patients in each treatment group. For patients able to be evaluated who had atypical pathogens, 18 of 18 patients (100%) in the GFX and 23 of 26 patients (88%) in the CLA group had a successful clinical outcome. There were similar rates of adverse events in each group, resulting in </= 7% withdrawal from treatment; gastrointestinal events were the most common. GFX, 600 mg qd, was equivalent to CLA, 500 mg bid, in treating adult patients with CAP. Both treatments were well tolerated.