HANOI, Vietnam -- The Vietnam War ended 25 years ago in April, but for Ngo

Bao Thang, the battle continues.

The 58-year-old North Vietnamese veteran fought for five years in the former

South Vietnam, where U.S. forces sprayed the defoliant Agent Orange to

destroy jungle ground cover for North Vietnamese troops.

After the spraying, ''it was difficult for us to open our eyes,'' Ngo says.

''Then the forest became empty.''

Ngo has five children, two of whom are mentally disabled. The Vietnamese

government suspects the disabilities were caused by Ngo's exposure to Agent

Orange.

No one really knows whether that is true. Agent Orange dioxin, considered

among the world's most toxic substances, has been linked to numerous

cancers, several other illnesses and one birth defect, spina bifida, says

the U.S. Institute of Medicine, which is under the National Academy of

Sciences.

Research in Vietnam has been piecemeal. However, momentum is building for

joint U.S. and Vietnamese studies that could help Vietnamese, exposed U.S.

veterans and people worldwide.

''Vietnam is the only country where so many people have been exposed to

dioxins at fairly high levels,'' says Chris Hatfield, president of Hatfield

Consultants, which is studying one heavily sprayed area. ''What is learned

here could help set more accurate standards for the world.''

Darkest aspects of war

The prospect of joint research got a boost Monday when U.S. Defense

Secretary William Cohen, in Vietnam for the first Defense secretary visit

since the war, reiterated U.S. willingness to do joint research. Last month,

Vietnamese officials assured a delegation from the Vietnam Veterans of

America that they also were ready to talk.

If research is done, it will look into one of the darkest aspects of the

Vietnam War. From 1961 to 1971, U.S. forces sprayed 20 million gallons of

herbicide, 60% of it Agent Orange, over more than 10% of South Vietnam. The

spraying, covering an area about the size of Vermont, was especially heavy

near the North Vietnam border and along the Ho Chi Minh Trail. The herbicide

was so powerful it killed mangrove forests and thick jungles. The impact is

still evident. Tenacious grasses cover large swaths of land where jungles

used to grow.

Agent Orange, named after the orange bands around the 55-gallon drums, was

meant to destroy plants, not to harm people. The dioxin was accidentally

created in the mixing of two herbicides, both of which were widely used in

the United States. The spraying ended after animal testing linked one of

Agent Orange's components to deformed fetuses.

Dioxins, mainly byproducts of industrial processes, exist worldwide. They

are mainly found in soil, dairy products, meat and fish.

The Vietnamese say that millions of people were exposed to Agent Orange and

that hundreds of thousands have since suffered from cancers, other illnesses

and birth defects. U.S. veterans have made similar claims. In 1984, they

settled a class-action lawsuit against the chemical makers, including Dow

and Monsanto. Veterans in South Korea also link illnesses to Agent Orange

spraying along the North Korean border in the late 1960s.

Industrial accidents in Europe also have exposed workers to high dioxin

levels. But only in Vietnam have so many people been exposed over such a

long period, says Arnold Schecter, a University of Texas dioxin expert who

calls Vietnam home to the world's largest dioxin contamination.

Since 1984, he has found people in the south of Vietnam with dioxin levels

higher than people in the north, which wasn't sprayed.

Hatfield, of the Canadian environmental research firm, spent five years

researching a single area of Vietnam. He also found high dioxin levels in

people born after the war. That indicates that the dioxin is ''still moving

from the soils to the food chain (especially fish) to people,'' he says.

'Hot spots'

His research indicates that Vietnam might have hundreds of ''hot spots''

where Agent Orange was heavily sprayed, accidentally spilled or applied

extensively to keep ground cover low around former U.S. military bases.

The Vietnamese, too, have conducted surveys that found higher cancer and

birth-defect rates among Vietnamese living near former U.S. military bases

and higher birth-defect rates among families of exposed veterans.

The studies, small and often lacking rigorous controls, are ''at best

suggestive'' that Agent Orange exposures have led to higher cancer and

birth-defect rates, Schecter says. Research is much stronger in linking

dioxin to cancer than to birth defects, he adds. Polio and malnutrition,

which was widespread in Vietnam for decades, might be responsible for many

of the malformations the Vietnamese often associate with Agent Orange, he

says.

He agrees that more studies are needed. In the past, efforts to conduct

joint research have failed. Vietnamese and U.S. researchers have feared that

highlighting the issue could hurt Vietnam's rice and other agricultural

exports, even though plants do not absorb dioxin.

The State Department says the United States has offered to carry out joint

studies for several years, although advocates say Washington has been deeply

concerned about possible litigation from any findings.

''The biggest fear of our government is compensating Vietnamese victims, and

the biggest fear of the Vietnamese is that it would shut down their country

in terms of business,'' says Thomas Corey, vice president of the Vietnam

Veterans of America.

Even if more studies are done in Vietnam, they should not delay action from

being taken in known contaminated areas, Hatfield says. The environmental

research firm, working with the Vietnamese government, recommended recently

that 12 families be moved from their homes near a former U.S. military

airbase, which Hatfield says has dioxin-contaminated soil. Fish ponds, too,

should be closed, he says. Residents are being told not to eat the fatty

tissue of fish and ducks, where dioxins concentrate. He says a rapid

assessment of other suspected hot spots in Vietnam should be a top priority.

More studies, though would do little to help those already exposed. Ngo's

family lives in a two-room house with a dirt floor. The farmer scratches out

a living. Vietnam has few homes for the disabled and meager financial help

for suspected Agent Orange victims. Last month, it set up its first $700,000

fund for veterans and their families.

Ngo's biggest worry is who will take care of his disabled children when he

and his wife cannot.

Is Agent Orange to blame? ''I don't know,'' he says. ''We just called it the

thing that destroyed the forests.''