Estimating the amount and cost of excess antibiotic use in ambulatory practice and identifying the conditions that account for most excess use are necessary to guide intervention and policy decisions. Data from the 1998 National Ambulatory Medical Care Survey, a sample survey of United States ambulatory physician practices, was used to estimate primary care office visits and antibiotic prescription rates for acute respiratory infections. Weight-averaged antibiotic costs were calculated with use of 1996 prescription marketing data and adjusted for inflation. In 1998, an estimated 76 million primary care office visits for acute respiratory infections resulted in 41 million antibiotic prescriptions. Antibiotic prescriptions in excess of the number expected to treat bacterial infections amounted to 55% (22.6 million) of all antibiotics prescribed for acute respiratory infections, at a cost of approximately $726 million. Upper respiratory tract infections (not otherwise specified), pharyngitis, and bronchitis were the conditions associated with the greatest amount of excess use. This study documents that the amount and cost of excessive antibiotic use for acute respiratory infections by primary care physicians are substantial and establishes potential target rates for antibiotic treatment of selected conditions.