A US court has been hearing an appeal by Vietnamese plaintiffs about the use of Agent Orange by American forces during the Vietnam War.
Supporters of alleged victims protested outside court
They hope to overturn a 2005 ruling by a lower court that decided there was no proof Agent Orange caused ill health.
The lawsuit, brought on behalf of more than three million Vietnamese, is seeking compensation from companies that manufactured the chemical.
The hearing comes as Vietnam's leader makes a landmark visit to the US.
President Nguyen Minh Triet is first Vietnamese head of state to make the trip to the US since the war ended in the 1970s.
The court was told he was expected to discuss the Agent Orange case and the effects of the chemical with US President George W Bush when they meet later in the week.
Agent Orange was the nickname given to a herbicide sprayed by US forces to destroy the jungle cover used by their enemies during the Vietnam War.
It contained the highly toxic chemical, dioxin.
Herbicide used to clear vegetation, denying enemy forces cover
Name derives from orange markings on the drums the chemical was shipped in
Children born in areas sprayed have disproportionate rate of mental and physical problems
According to Vietnamese victims' groups, the herbicide caused at least three million disabilities and birth defects.
The plaintiffs have brought their action against 37 US producers and suppliers of Agent Orange, including Dow Chemical and Monsanto.
The plaintiff's lawyer, Jonathan C Moore, told the appeal hearing that the companies knew the herbicide contained the harmful dioxin but did nothing about it.
"They knew how it was going to be used, and they had reason to believe the effect would be disastrous and they did it anyway," he said.
"We are now seeing years later the fruit of that terrible poisonous product."
But Seth Waxman, a lawyer acting on behalf of the companies, told the court that they were following the instructions of the US president and government during wartime.
"Even an international tribunal would not entertain these claims," he said.
The three-judge panel must now decide whether to overturn a federal court ruling in 2005 that the plaintiffs had failed to prove the chemicals caused genetic defects.
The decision could take several months.
Before the hearing started, victims and their supporters gathered outside the courthouse in New York for a rally.
"We need to tell the American citizens of the bad impact and consequences of Agent Orange to many generations in Vietnam," said Quy, who had travelled from Vietnam for the hearing.
Recent studies have found levels of the chemical at former storage sites in Vietnam to be many times higher than international-accepted limits.
The US has offered money to clean-up contaminated sites and has also spent $43m on helping Vietnamese with disabilities.
However, it has rejected claims for compensation from alleged Agent Orange victims, maintaining that there is a lack of proof that the chemical caused people's disabilities.