Five hundred four patients were enrolled in a randomized, double-masked, multicenter study comparing the efficacy and tolerability of a 10-day regimen of sparfloxacin with a 14-day regimen of clarithromycin in the treatment of acute maxillary sinusitis. Two hundred fifty-two patients received sparfloxacin as a single 400-mg dose on day 1 and 200 mg once daily for 9 additional days, and 252 patients received clarithromycin 500 mg twice daily for 14 days. In the all-treated population, clinical success was observed at 6 to 10 days after therapy in approximately 82% of the patients in each treatment group. A total of 430 patients met the inclusion criteria for clinical assessment. The success rates in these patients were also comparable, at 83.1% and 83.4% for the sparfloxacin and clarithromycin groups, respectively. Sustained clinical success rates in the all-treated population 3 to 4 weeks after therapy were 71.6% for the sparfloxacin group and 68.6% for the clarithromycin group. All treated patients were included in the tolerability analysis. The frequency of adverse events in the clarithromycin and sparfloxacin groups was 57.9% and 48.4%, respectively. The most frequently noted adverse events were diarrhea, photosensitivity reaction, taste perversion, nausea, and abdominal pain; >96% of adverse events in the sparfloxacin group and 94% of adverse events in the clarithromycin group were of mild or moderate severity. Among adverse events at least possibly related to study drug, photosensitivity reaction was more common in the sparfloxacin group (9.5% vs. 0.4%), whereas taste perversion (8.7% vs. 0.8%) and abdominal pain (3.6% vs. 1.6%) were more common in the clarithromycin group. Thus the sparfloxacin's more convenient regimen was as effective as clarithromycin in the treatment of acute bacterial maxillary sinusitis, and the overall frequency of adverse events with sparfloxacin was comparable to that with clarithromycin.