The further seriously ill patients have to travel by ambulance to reach emergency care, the more likely they are to die, reveals research in Emergency Medicine Journal.
People with respiratory problems seem to be at greatest risk, the study indicates.
The findings have implications for the UK government's proposals to close local emergency care departments in favour of fewer more specialised centres, in a bid to save lives, say the authors.
Local closures will inevitably spell longer ambulance journeys for critically ill patients, they say.
The findings are based on a review of life-threatening (category A) calls to four ambulance services in England, representing urban, rural, mixed, and remote areas, between 1997 and 2001.
Only those patients who were unconscious, or not breathing, or who had chest pain were included in the study.
Almost 12,000 patients fell into this category over the five years, but the final analysis included 10,315 as not all the distances could be calculated accurately.
Distances covered ranged from zero to 58 kilometres, but the average was 5 km.
In all, 644 patients (just over 6%) died during the five year period, but the further patients had to travel by ambulance to hospital, the more likely they were to die.
In all, the risk of death rose by 1% for every 10 kilometres (6 miles).
But compared with patients with chest pain, injuries/poisoning, or other types of trauma, those with breathing problems were most at risk.
Their chances of dying were 13% if the distance to hospital was between 10 and 20 km, and 20% if this was 20 or more kilometres.
The findings held true even after taking account of age, sex, and illness severity.
"Our data suggest that any changes that increase journey distances to hospital for all emergency patients may lead to an increase in mortality for a small number of patients with life-threatening medical emergencies, unless care is improved" conclude the authors.