The relationship between drug exposure and the time course of antimicrobial effect at the primary infection site for acute maxillary sinusitis has not previously been explored. This single-center, open-label study quantified the time course of sinus sterilization, described gatifloxacin exposure at the infection site, and posed the hypothesis that the use of continuous and quantitative time-related end points may allow for better characterization of drug effect with fewer patients than traditional clinical trial approaches. Of the 12 enrolled patients, 10 were clinically evaluable, from whom 7 pathogens were isolated: 4 Streptococcus pneumoniae, 2 staphylococci, and 1 Enterobacter aerogenes. The median predicted 24-h area under the curve (AUC) in sinus aspirates and plasma samples was 54.7 mg x h/L and 30.1 mg x h/L, respectively. The median 24-h AUC ratio for sinus aspirates and plasma samples was 1.51 (range, 0.88-2.23). For patients infected with pneumococci, the median time to sinus sterilization was 50 h. The use of quantitative time-related end points may be useful in evaluating the efficacy of antimicrobial agents with fewer patients.