OBJECTIVES: To assess the different types of antibiotic-taking behavior and to compare self-reported with objectively measured adherence to antibiotic regimens in respiratory infections. METHODS: This was a prospective study of patients with suspected bacterial pharyngitis and lower respiratory tract infections recruited from five primary care clinics in Catalonia. Adherence to various antibiotic regimens was assessed by the Medication Event Monitoring System (MEMS), which recorded every opening of the patient's bottle of tablets, and a self-reported adherence question. The outcome variables were antibiotic-taking adherence, correct dosing, and timing adherence. RESULTS: A total of 428 patients were included in the analysis. Five types of antibiotic use behavior were observed: excellent adherence (130 patients, 30.4%), acceptable adherence over time (53; 12.4%), declining adherence over time (123; 28.7%), non-adherence to correct dosing (108; 25.2%), and unacceptable adherence (14; 3.3%). Excellent adherence was significantly associated with the number of daily doses of antibiotic and antibiotic duration. A total of 254 patients reported never forgetting to take the antibiotic (59.3%), achieving a negative predictive value of 100% and a positive predictive value of 51.2%. CONCLUSIONS: Outpatients with respiratory infections treated with antibiotics showed poor adherence outcomes. Self-reported adherence was remarkably higher than that observed with the use of MEMS and failed to predict true patient adherence.