Antibiotic treatment of respiratory tract infection (RTI) has been evaluated in many clinical trials, but little is known about its effectiveness (efficacy in real life conditions) and costs. OBJECTIVE: To measure the effectiveness, side effects and costs associated with the use of various antibiotics in the ambulatory treatment of RTI in adults. METHODS: Patients aged 18+ were recruited by community pharmacists. They were considered for inclusion if they reported being treated for an RTI and received a new prescription for one of the following antibiotics: amoxicillin, amoxicillin/clavulanate, azithromycin, cefaclor, cefuroxime, ciprofloxacin, clarithromycin or erythromycin. Presence of symptoms, side effects and resources utilization were reported in a self-administered questionnaire covering the first 14 days after treatment was initiated. A follow-up call at 28 day completed the data collection. Resources utilization considered included initial antibiotic, physician and pharmacist services, other medications (prescribed and OTC), and alternate antibiotic when required. RESULTS: Of 989 patients recruited, 777 returned their questionnaire (79%). At day 7, 33% of patients reported having symptoms. This proportion did not vary according to the antibiotic even when controlling for the type of RTI (URTI vs. LRTI) (?2, p>0.05). Diarrhea was reported by 17%, nausea by 11% and abdominal pain by 10% of patients. Erythromycin had the largest proportion of patients reporting a GI side effect (53%). Average cost per episode varied between $34 and $89 (Canadian). Lower costs were seen for patients started with amoxicillin or erythromycin. The highest cost was seen in patients started with cefaclor/cefuroxime. CONCLUSION: Effectiveness was not dependent on the antibiotic selected at the start of treatment. Occurrence of side effects was relatively high.