Gulf war veterans are more likely to develop motor neurone disease, according to US studies.
It adds to concerns that former servicemen may have been exposed to something during the 1991 conflict which increased their risk of illness.
The studies, published in the journal Neurology, found that veterans of the war were three times more likely to develop the disease.
The illness leads to progressive muscle weakness and eventually death.
Veterans in the UK and US have reported a variety of symptoms which they blame on exposure either to vaccines given prior to deployment, or some other environmental factor they encountered during the campaign.
Preliminary results from one of the latest studies had already provided enough evidence of the motor neurone link for US authorities to classify the illness as "attributable to service" and offer compensation.
However, now the results have been published in full - alongside another study that backs these findings.
Three times risk
In this, researchers from The University of Texas Southwestern Medical Center at Dallas looked at statistics on the rates of a type of motor neurone disease called amyotrophic lateral sclerosis - known in the US as Lou Gehrig's disease.
"
While a twofold increase in risk may seem impressive, one needs to realise that this is based upon a just a small number of cases
"
Dr Michael Rose, Kings College Hospital, London
They found that rates among the veterans were three times the rates expected in such relatively young people.
Dr Robert Haley, who led the study, said: "The disease occurred in a very abnormal age group - in people in their 20s and 30s instead of 60s and 70s.
"It raises the question whether the condition might have been caused - or triggered prematurely - by unusual environmental exposures in the war."
The other study, by the National Institute of Neurological Disorders and Stroke, suggested a two-fold increase in the rate of motor neurone diseases among veterans.
Rare event
The problems with the study arise from the fact that motor neurone is extremely rare in under 45s - so even a small number of cases would represents a substantial increase in risk.
However, Dr Michael Rose, from King's College Hospital in London, said: "While a twofold increase in risk may seem impressive, one needs to realise that this is based upon a just a small number of cases.
"Therefore the calculated risk may easily be changed either way if the methodology has any flaws."
Shaun Rusling, the Chairman of the National Gulf Veterans and Families' Association, said that he was aware of six cases of motor neurone disease in relatively young veterans of the conflict.
He said: "When you consider that I have been told that this disease affects only one in 250,000 people, that is very unusual and worrying.
"However, the Ministry of Defence has simply fobbed us off about it."
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