According to the NHS Confederation (2012), “It’s not good to be a patient stuck on an ambulance trolley in a corridor, but the patients at risk are the road traffic accidents or heart attacks still waiting for the ambulance service to respond”. Ambulance Turnaround time is a measure of particular interest in the performance of Emergency Medical Systems (EMS). It determines the degree of coordination and communication with the health institutions where each patient will receive his final treatment.  Its value is key in the determination of the ambulance capacity of the operations system of an EMS institution. Long turnaround times decreases the availability of ambulances for future services and furthermore, it increases ambulance response times and patient´s health risk. This work suggests an improvement approach based on Lean Thinking for reducing ambulance turnaround time. The approach is applied to a Mexican EMS institution based on metropolitan Monterrey. Results of the application are provided.