During the summer seasons of 1978-1983 (56 weeks), trial activity with a physician-manned ambulance helicopter was carried out in the County of Stockholm (population 1.5 million). The area includes an extensive archipelago. Within the region there are 11 large general hospitals, three of which are supplied with a helicopter pad. During the trial period 1 246 helicopter missions were undertaken, of which 943 resulted in emergency transportation of the patient (83% by helicopter, 17% by ambulance). The number of missions per day varied from 0-11, with a mean number of 3.2. The turn-out time from receipt of the call to arrival at the scene of the accident or illness was less than 30 minutes in 75% of the cases. Of the emergency calls, 57% involved cases of illness and 43% accidents. Therapeutic intervention by doctors at the scene was considered to have been of vital importance in 22% of the cases, desirable in 28% and unnecessary in 50%. This intervention was regarded as life-saving in 18 cases (2%). By special training of emergency call operators, the number of unnecessary helicopter missions could be minimised (9% of all missions). To continuing specialisation and centralisation of emergency care with extended catchment areas, demands an advanced transport system. This trial has shown that through the use of a physician-manned ambulance helicopter the emergency care within the region can be centralised with retention of adequate turn-out times.