OBJECTIVES: Evaluate costs of ambulatory treatment of lower respiratory tract infection (LRTI) in seniors. METHODS: Using claims data from Quebec public provincial health programs for physician services and prescriptions dispensed, we drew a sample of seniors diagnosed with LRTI in an ambulatory setting subsequently receiving an antibiotic from a community pharmacy. Failure of antibiotic was defined as receipt of a second antibiotic or inpatient physician visit with LRTI diagnosis within a 15-day window following antibiotic receipt. Costs of treatment (CDN$) were those associated with treatment episode and included medical visits, antibiotics, and hospitalisations associated with LRTI diagnosis. Treatment episode started with the initial visit and terminated 15 days after end of recorded duration of antibiotic received, extended by treatment duration (plus 15 days) of any additional antibiotics received during this period. The episode ended if an inpatient visit was recorded. Hospitalisation costs were estimated according to age-group CMG-specific resource intensity weights. RESULTS: Of 10565 subjects, 10.1% were deemed to be treatment failure. For subjects without failure, mean treatment cost was $69.95 (SD=20.70) compared with $515.81 (SD=911.03). The results of multiple regression on log-transformed total costs using as independent variables the type of antibiotic initially prescribed, age group, gender, diagnosis and chronic disease score, suggest that age 75+, diagnosis of pneumonia (contrasted to bronchitis), and a chronic disease score of 4+ are associated with increased total cost. Use of cephalosporins, clarithromycin 500 and quinolones were associated with higher total cost than use of azithromycin 250mg. Use of clarithromycin 250, erythromycin, amoxicillin, other penicillins, trimethoprim-sulfas and other antibiotics were associated with lower total cost than use of azithromycin 250mg. CONCLUSIONS: Total cost of treatment of LRTI in seniors is associated with older age, high disease score, and type of antibiotic used.