Background Oral chlorhexidine prophylaxis can decrease occurrence of ventilator-associated pneumonia. However, the importance of timing has never been fully explored. Objective To see if early administration of oral chlorhexidine is associated with lower incidence of early ventilator-associated pneumonia (within 5 days of admission to intensive care unit) in intubated air ambulance patients. Methods A single-center, retrospective cohort study of intubated adults transported by a university-based air ambulance service and admitted to a surgical intensive care unit from July 2011 through April 2013. Primary exposure was time from helicopter retrieval to the first dose of oral chlorhexidine in the intensive care unit. Early chlorhexidine was defined as receipt of the drug within 6 hours of helicopter departure. The primary outcome was clinical diagnosis of early ventilator-associated pneumonia. Patients who were less than 18 years old, died within 72 hours of admission, or had pneumonia at admission were excluded. Results Among 134 patients, 49% were treated with chlorhexidine before 6 hours, 84% were treated before 12 hours, and 11% were treated for early pneumonia. Early chlorhexidine (before 6 hours; 15%) was not associated (P = .21) with early pneumonia (8%). Furthermore, median times to chlorhexidine did not differ significantly (P = .23) between patients in whom pneumonia developed (5.2 hours) and patients with no pneumonia (6.1 hours). Conclusions Early administration of oral chlorhexidine in intubated patients was not associated with a reduction in the incidence of ventilator-associated pneumonia in a surgical intensive care unit with high rates of chlorhexidine administration before 12 hours.