In a government study that followed 4000 participants over a 30-year period, researchers found that 90% of men and 70% of women were overweight or later became overweight.
"National surveys and other studies have told us that the United States has a major weight problem, but this study suggests that we could have an even more serious degree of overweight and obesity [cases] over the next few decades," said Elizabeth Nabel, director of the US National Heart, Lung, and Blood Institute.
Excess weight and obesity increase the risk of poor health and can lead to diabetes, high blood pressure, heart disease, stroke, breathing problems and some cancers, Nabel said.
"We hope these results will serve as a wake-up call to Americans of all ages."
The study's results, which were published in this week's issue of the Annals of Internal Medicine, were based on assessments of body mass index, a standard measure of weight relative to height.
The volunteers for the study were white, from the town of Framingham in the northeastern state of Massachusetts.
Researchers said their study might underestimate the problem among Hispanic and African-American communities, which tend to have higher rates of excess weight.
According to the government's National Centre for Health Statistics, an estimated 65% of US adults over the age of 20 are either overweight or obese, with 30% of adults considered obese.
"We hope these results will serve as a wake-up call to Americans of all ages"
US National Heart, Lung, and Blood Institute director
The study also found that making it to middle age at a healthy weight was no guarantee for staying at that weight.
About one in five women and one in four men in the study deteriorated during a four year period, gaining excess weight or becoming obese.