US makers of the Agent Orange defoliant used in the Vietnam War should compensate a group of South Korean troops it affected, a Seoul court said.
Thousands of South Korean troops complain they were affected
The court said Dow Chemical and Monsanto should pay $62m in medical compensation to 6,800 people.
It was the first South Korean court ruling in favour of those affected.
South Korea sent more than 300,000 troops to fight in the 1965-73 Vietnam conflict, the largest outside contribution to the US war effort.
The High Court ruling, which overturned a lower court's decision, is part of a long-running case in which more than 20,000 South Korean veterans originally sued for damages.
On Thursday, the court said Agent Orange manufactured by the two US companies contained dioxins in excess of permitted levels.
A lawyer for Dow Chemical said the company was likely to appeal, according to the AP news agency.
It was not clear what effect, if any, the ruling would have on other legal action over Agent Orange.
Last year, a US court rejected a claim for damages from a group of Vietnamese who said they had suffered poor health because of the defoliant.
The chemical companies defending that case - including Dow Chemical and the Monsanto Corporation - argued that the US government was responsible for how the chemical was used, not the manufacturers.
This was despite a 1984 deal whereby several chemical companies paid $180m to settle a lawsuit brought by US war veterans, who said that their health had been affected by exposure to the substance.
Agent Orange was used to destroy forest cover and undergrowth which shielded Communist North Vietnamese troops from view.
It was named after the colour of its container, and as well as herbicides which stripped trees bare, it contained a strain of dioxin.
After the war ended, Vietnamese, US and South Korean soldiers and civilians claimed it had severe medical effects.
In Vietnam, some contend, the dioxin entered the food chain and caused a proliferation of birth defects.
Some babies were born without eyes or arms, or were missing internal organs.
Activists say three million people were exposed to the chemical during the war, and at least one million suffer serious health problems today.