INTRODUCTION: Ketamine is a dissociative agent used for sedation and intubation in various clinical settings. Despite its proven haemodynamic safety, ketamine has not been widely used in prehospital medicine. This study examined the use of ketamine in helicopter emergency medical services (HEMS). METHODS: This prospective cohort study enrolled all patients transported by a single HEMS program in whom ketamine was used to facilitate intubation. Data were collected using standard forms by two independent trained research staff. Demographics, medical condition, intubation conditions, vital signs (pre and post drug administration) and complications were recorded. Proportions, medians with IQR, change scores and CIs are reported; differences were compared using paired t tests. RESULTS: During the 2-year study period, 71 patients received ketamine to facilitate endotracheal intubation. Ketamine was used most often in men (52 (73%)), and the median age was 49 years (IQR: 31, 69). Most patients were adults (70 (99%)) with medical illnesses (42 (59%)); 37 (52%) intubations were performed at the sending hospital, and 30 (42%) were performed on scene. A paramedic performed the intubation in 58 cases (82%). The median ketamine dose was 80 mg (IQR: 60, 100; ~ 1mg/kg); 53 (75%) patients also received a paralytic agent. Mean arterial pressure (2.3 mmHg; 95% CI: -8.0 to 3.3) and heart rate (0.45 beats/min, 95% CI: -4.9 to 4.0) changes failed to reach statistical or clinical significance. No differences were found between patients with suspected concomitant head injury and other patients with respect to ketamine dose, changes in vital signs and complications. Complications included: one (1.4%) interstitial IV, five (7%) failed intubations, five (7%) hypotension and four (6%) hypertension episodes, one (1%) bradycardia, two (3%) tachycardia and five (7%) deaths. CONCLUSIONS: Ketamine is an effective agent in facilitating intubation in a HEMS environment. Complications are similar to use in the controlled Emergency Department setting.