A study from the Institute of Medicine released Friday contains several caveats, but suggests there is a stronger connection than previously thought about the health risks to Vietnam veterans.
The research was sponsored by the Veterans Affairs Department, which will decide what to do with the findings.
American forces sprayed millions of gallons of Agent Orange and other defoliants over parts of Vietnam from 1962 to 1970. Military authorities used the defoliants in an attempt to massively prune away the dense jungle cover used by Communist forces to hide.
American troops and others exposed to the chemicals later complained of numerous health problems, however, and researchers are still trying to determine the scope of the damage.
The Institute of Medicine, a division of the National Academy of Sciences, is mandated by Congress to review every two years evidence about the effects of Agent Orange exposure.
To determine whether Vietnam veterans faced an increased chance of ischemic heart disease - a condition involving reduced blood supply to the heart - researchers reviewed several studies that showed links between higher exposure levels and greater incidence of the disease.
Other factors such as smoking, age, and weight can also play a role, they noted. Still, they said veterans exposed to the chemicals may be at greater risk.
The conclusion on Parkinson's was based on a review of 16 studies that looked at herbicide exposures among people with the disease or Parkinson's-like symptoms. But the study cautions the review was hindered by the lack of studies specifically investigating Parkinson's rates among Vietnam veterans.