Background: Judicious use of antibiotics can slow the spread of antimicrobial resistance. However, overall patterns of antibiotic use among ambulatory patients are not well understood. Objective: To study patterns of outpatient antibiotic use in the United States, focusing on broad-spectrum antibiotics. Design: Cross-sectional survey in three 2-year periods (1991-1992, 1994-1995, and 1998-1999). Setting: The National Ambulatory Medical Care Survey, a nationally representative sample of community-based outpatient visits. Patients: Patients visiting community-based outpatient clinics. Measurements: Rates of overall antibiotic use and use of broad-spectrum antibiotics (azithromycin and clarithromycin, quinolones, amoxicillin-clavulanate, and second- and third-generation cephalosporins). All comparisons were made between the first study period (1991-1992) and the final study period (1998-1999). Results: Between 1991-1992 and 1998-1999, antibiotics were used less frequently to treat acute respiratory tract infections, such as the common cold and pharyngitis. However, use of broad-spectrum agents increased from 24% to 48% of antibiotic prescriptions in adults (P < 0.001) and from 23% to 40% in children (P < 0.001). Use of broad-spectrum antibiotics increased across many conditions, increasing two- to threefold as a percentage of total antibiotic use for a variety of diagnoses in both adults and children. By 1998-1999, 22% of adult and 14% of pediatric prescriptions for broad-spectrum antibiotics were for the common cold, unspecified upper respiratory tract infections, and acute bronchitis, conditions that are primarily viral. Conclusions: Antibiotic use in ambulatory patients is decreasing in the United States. However, physicians are increasingly turning to expensive, broad-spectrum agents, even when there is little clinical rationale for their use.