A multicenter, investigator-blinded, randomized, parallel-group study was conducted to compare oral levofloxacin 500 mg once/day for 14 days with clarithromycin 500 mg twice/day for 14 days in the treatment of acute bacterial sinusitis. Of 216 adult outpatients randomized to treatment, 190 were evaluable for efficacy. The primary efficacy measure was clinical response, based on resolution of signs and symptoms 2-5 days after therapy. A secondary efficacy measure was relapse rate 1 month after therapy. Among evaluable patients, clinical success rates (cured or improved) were 96.0% and 93.3% for levofloxacin (L) and clarithromycin (C), respectively (95% CI -9.2%, 3.7%). The confidence interval (CI) for treatment difference (C-L) included zero and its upper limit was less than 15%, indicating that levofloxacin was as effective as clarithromycin. In all, 4.1% of patients receiving levofloxacin and 7.2% receiving clarithromycin had a relapse of symptoms 1 month after therapy (95% CI -12.2%, 3.2%). Long-term success (initial success, absence of relapse at 1 month, no further antibacterial therapy 2-5 days after therapy) was 79.2% in the levofloxacin group and 76.4% in the clarithromycin group (95% CI -14.7%, 9.0%). Based on investigator-assessed treatment-emergent adverse events, overall tolerability of the drugs was similar, except for a higher frequency of taste perversion and diarrhea in the clarithromycin group. Levofloxacin had an advantage over clarithromycin based on two quality-of-life (QOL) parameters: number of times taking other drugs for targeted medical conditions and mean total cost of these drugs. No statistical significance was found in other QOL variables. These findings suggest that the efficacy and tolerability of levofloxacin 500 mg once/day are comparable with those of clarithromycin 500 mg twice/day in the treatment of acute bacterial sinusitis.