This contradicts previous research which found that the genetic configurations underpinning the two autoimmune disorders were mutually exclusive, suggesting that type-1 diabetes and MS should therefore not co-occur.
The researchers used data from two population-based registers of patients in Denmark to see whether type-1 diabetes and MS did co-occur, and to what extent.
Using a national hospitalisation register, they identified 6,078 patients diagnosed with type-1 diabetes in 1977.
They then searched for these patients in a national register of 11,862 MS patients to see if any of them went on to develop MS over the following 20 years.
The researchers found that 11 of the type-1 diabetes patients went on to be diagnosed with MS. This is more than three times as many as would be expected based on general population figures.
On average, these patients were diagnosed with type-1 diabetes aged 13 years and with MS at 25 years.
The risk of developing MS was found to be highest in those diagnosed before age 15.
The researchers then looked at data from 14,771 first-degree relatives of the patients with MS to see if having a familial risk of MS was associated with an increased risk of developing type-1 diabetes.
They found that family members of MS patients had a 40 per cent increased risk of developing type-1 diabetes compared to the general population.
The researchers, led by Dr Nete Nielsen, of the Department of Epidemiology Research, Statens Serum Institut in Copenhagen, concluded that MS and type-1 diabetes can co-occur.
'Analysis showed that patients with type-1 diabetes are at a threefold risk for development of MS, and that the risk for type-1 diabetes in first-degree relatives of patients with MS is increased by approximately 40 per cent.
'The underlying mechanisms remain unknown, but may involve both genetic and environmental factors,' they concluded.
Archives of Neurology 2006; 63: 1,001-4