CONTEXT: In critically ill intubated patients, signs of respiratory infection often persist despite treatment with potent systemic antibiotics. OBJECTIVE: The purpose of this study was to determine whether aerosolized antibiotics, which achieve high drug concentrations in the target organ, would more effectively treat respiratory infection and decrease the need for systemic antibiotics. DESIGN: Double-blind, randomized, placebo-controlled study performed from 2003 through 2004. SETTING: The medical and surgical intensive care units of a university hospital. PATIENTS: Critically ill intubated patients were randomized if: 1) > or = 18 yrs of age, intubated for a minimum of 3 days, and expected to survive at least 14 days; and 2) had ventilator-associated tracheobronchitis defined as the production of purulent secretions (> or = 2 mL during 4 hrs) with organism(s) on Gram stain. Of 104 patients monitored, 43 consented for treatment and completed the study. No patients were withdrawn from the study for adverse events. INTERVENTION: Aerosol antibiotic (AA) or aerosol saline placebo was given for 14 days or until extubation. The responsible clinician determined the administration of systemic antibiotics (SA). Patients were followed for 28 days. MAIN OUTCOME MEASURES: Primary: Centers for Disease Control National Nosocomial Infection Survey diagnostic criteria for ventilator-associated pneumonia (VAP) and clinical pulmonary infection score. Secondary: white blood cell count, SA use, acquired antibiotic resistance, and weaning from mechanical ventilation. RESULTS: Most patients had VAP at randomization. With treatment, the AA group had reduced signs of respiratory infection: reduced Centers for Disease Control National Nosocomial Infection Survey VAP (14/19; 73.6%) to (5/14; 35.7%) vs. placebo (18/24; 75%) to (11/14; 78.6%), reduction in clinical pulmonary infection score, lower white blood cell count at day 14, reduced bacterial resistance, reduced use of SA, and increased weaning (all p < or = .05). CONCLUSIONS: In critically ill patients with ventilator-associated tracheobronchitis, AA decrease VAP and other signs and symptoms of respiratory infection, facilitate weaning, and reduce bacterial resistance and use of systemic antibiotics.