Hillary Clinton is due in Laos as the first US secretary of state to visit the country in 57 years.
She is expected to discuss plans to build a controversial Mekong River dam, which critics say would impact the environment and millions of lives.
The effects of Agent Orange, a defoliant used during the Vietnam war, are also likely to be on the agenda.
Clinton, who arrives in Vientiane from Vietnam, will then head to Cambodia for an Asean meeting.
The last top US diplomat to visit Laos was John Foster Dulles in 1955.