OBJECTIVES To observe the frequency with which children in outpatient primary care settings are prescribed antibiotics and to investigate why these antibiotics are prescribed. To compare the prescribing behaviour of family doctors, primary care pediatricians, and urgent care physicians and to determine where refinements in management are most needed to reduce the number of antibiotic prescriptions appropriately. DESIGN Prospective study using a data entry form with mostly closed-ended questions. SETTING Ten primary care offices in urban southcentral and eastern Ontario: five family practices, three pediatric practices, and two urgent care centres (UCC). PARTICIPANTS Every child younger than 16 years visiting these offices during a 3-week period in February and March 1997. MAIN OUTCOME MEASURES Frequency, clinical indications, and nature of the antibiotics prescribed. RESULTS There were 4344 observed visits. Of 1706 antibiotic prescriptions, 1481 were for 10 days, and 1577 (92%,) were for acute respiratory infections, 920 (53%) specifically for acute odds media (AOM). Full courses of antibiotics were given immediately tie, without test results) to 321 (76% of 425 children with pharyngitis. Antibiotics were prescribed for 145 (90%) of 163 children with bronchitis. Urgent care physicians were significantly more likely than pediatricians or family physicians to prescribe immediate antibiotics and to disregard guidelines when choosing antibiotics for uncomplicated AOM. CONCLUSIONS Three diagnoses accounted for 82% of antibiotic prescriptions: AOM, pharyngitis, and bronchitis. Physicians should be more selective when deciding whether, and for how long, to prescribe antibiotics for those three common conditions. Substantial reductions in antibiotic use will require changes in how physicians manage suspected AOM, the most common indication for antibiotics.