OBJECTIVE: To examine antibiotic prescribing and outcomes associated with acute sinusitis in managed care. METHODS: We used the PharMetrics claims database for 8 managed care organizations. Index claims were based on outpatient visits in 1999-2001 by individuals aged 45-64 with a primary diagnosis of acute sinusitis who received an antibiotic within 7 days and were continuously enrolled for 12 months prior to and 45 days after the index event. Exclusion criteria: antibiotic prescription or sinusitis diagnosis in prior 45 days, hospitalization in prior 30 days, or sinus complications in prior 12 months. Broad spectrum antibiotics (BSA) were defined as: azithromycin, clarithromycin, amoxicillin-clavulanate, second- and third-generation cephalosporins, quinolones. Sinusitis history was categorized: chronic sinusitis (HxChr), acute but not chronic sinusitis (HxAcu), no history (NoHx). Also, subjects with asthma, chronic obstructive pulmonary disease (COPD), lower and upper respiratory tract infection (LRTI, URTI),and rhinitis in prior year were identified. Multivariate models adjusted for age, sex, health plan, use of laboratory/diagnostic testing at initial evaluation (surrogate for severity and practice variation). RESULTS: Out of 64,277 cases, 66.1% female, met criteria. Respiratory history: 5.4% HxChr, 7.9% HxAcu, 4.5% asthma, 4.4% COPD, 24.6% URTI, 16.3% LRTI, 10.4% rhinitis. BSA accounted for 45.5% of 1st-line prescriptions. BSA use increased over 3 years (p <0.0001):43.8% (1999), 45.5% (2000), 49.2% (2001). BSA use was highest for HxChr and asthma at 54.3%. Overall, 22.4% received a 2nd prescription, highest among HxChr, 31.7%. Mean charges were $134; highest for HxChr, $158 and asthma, $152. In multivariate models, respiratory histories were positively associated (p <0.05) with BSA use, 2nd prescriptions and charges. CONCLUSION: In treatment of acute sinusitis in managed care, broad spectrum antibiotics are used almost 50% of the time as initial therapy and use is rising. Respiratory history, especially chronic sinusitis or asthma, is associated with more BSA use, 2nd prescriptions, and charges.