The United States and Vietnam have signed an agreement to jointly research the damage that Agent Orange has done in Vietnam, more than 30 years after the U.S. military stopped spraying the defoliant.

Environmental officials from both countries signed the agreement Sunday, in a significant step toward burying what has been called the last ghost of the Vietnam War.

Vietnamese and U.S. researchers will start with joint studies of birth defects and cancer in areas of Vietnam known to have high levels of dioxin, the toxic chemical in Agent Orange. It could take years to complete the studies, and how much they will cost is still unclear.

Christopher Portier, the head of the American research team, says the cost will depend on the type of projects done. The bill could range from a few million dollars to $50 million, spent over 10 years.

Mr. Portier says the agreement will focus on three areas, including environmental sampling and remediation, and human health effects research. "And the third area is exchange of scientists, so Vietnamese scientists will work in the United States for a while and U.S. scientists will work here for a while," he said.

Vietnam estimates more than one million people have health problems caused by Agent Orange. The U.S. military sprayed the defoliant to kill trees and deny North Vietnamese troops jungle cover.

Agent Orange contained dioxin, which has been linked to cancer, birth defects, liver damage and a host of other maladies. Vietnamese researchers say that even now, 30 years after the spraying stopped, babies born in dioxin hot spots have three to four times the normal rate of birth defects.

But Washington doubts the accuracy of the Vietnamese studies and suspects Hanoi of classifying nearly every birth defect in the country as being caused by Agent Orange. The United States insists more studies are needed to determine the extent of the damage.

Vietnamese and American researchers already have identified the first dioxin hotspot for study, an airport in the central province of Danang.