The optimal extent of prehospital care, including intravenous (IV) therapy for critically ill patients, remains unclear. The authors evaluated the success rate for IV cannulation in a moving ambulance by trained emergency medical technicians and paramedics in 641 adult medical- and trauma-related cases. At least one IV line was started in 80% of medical patients and 92% of trauma patients, regardless of blood pressure. In hypotensive patients, the success rates for at least one IV in medical and trauma patients were 80% and 95%, respectively. These data suggest that IV lines can be secured with a high degree of success en route to the hospital by trained personnel, and that prompt transport of unstable patients should not be delayed solely to obtain IV access.