To document the rate and cost of antibiotic prescribing for patients diagnosed only with influenza during US ambulatory care visits. Federal survey data for 1997-2001 were used to estimate outpatient trends for all patients and healthy people age 5-49 years. Cost estimates were based on Medicare payments and Red Book average wholesale prices in 2003. Antibiotic prescribing for influenza is widespread; 38% of visits led to an antibiotic prescription of which one-third were for broad spectrum antibiotics. Inappropriate antibiotics cost dollar 18.5 million annually and may contribute to resistance. Increased vaccination rates and viral testing could reduce these trends.