The first study included data from 12,274 patients in Saskatchewan, Canada, with an average age of 64, who were newly diagnosed with type-2 diabetes.
In the five years following diagnosis, more than 9 per cent were admitted to hospital with stroke, twice the rate of the general population.
Other studies have shown an increased risk of stroke with type-2 diabetes, but this is the first to show how soon the increased risk kicks in.
Lead researcher Dr Thomas Jeerakathil, assistant professor of medicine and neurology at the University of Alberta in Edmonton, Canada, said the study showed the importance of early treatment to lower cholesterol and BP in patients with type-2 diabetes.
'One would think that the consequences of diabetes would occur over a long period but we found that new-onset diabetics have double the rate of stroke in the first five years after diagnosis,' he said.
'People with a new diagnosis should have all their cardiovascular risk factors managed optimally. That means strict control of BP and elevated cholesterol, avoiding a sedentary lifestyle and a healthy diet.'
The second study looked at the risk of stroke in patients of different ages with type-2 diabetes. It found that younger patients were at the highest risk of stroke, with those aged under 55 being six to 17 times more likely than a non-diabetic patient of the same age to have a stroke.
Black patients also had higher stroke risk than white patients.
Lead researcher Dr Brett Kissela, associate professor in the department of neurology at the University of Cincinnati in Ohio, said: 'This risk is especially prominent among young people who have diabetes diagnosed at an early age.'
He said GPs needed to be particularly diligent about treating high BP in this group of patients: 'I would argue that diabetes is the second biggest risk factor for stroke after hypertension.'
Chairman of the Primary Care Diabetes Society, Dr Colin Kenny, said GPs should focus on stroke prevention through treating hypertension.
'The focus on hypertension management in the quality framework is clearly important. But this suggests that all diabetes patients should perhaps also be receiving aspirin,' he said.