Nearly half of all personal bankruptcies in the United States are triggered by big medical bills racked up because of serious illnesses or accidents, a Harvard University study suggests.

The study, published Wednesday in the online journal Health Affairs, looked at 1,771 people who had declared personal bankruptcy to seek court protection from creditors in five American states in 2001.

Researchers from Harvard's law and medical schools later talked to 931 of them. They determined that illness or medical bills were either the main cause or a contributing factor in 46.2 per cent of the bankruptcies.

"Even middle-class insured families often fall prey to financial catastrophe when sick," an abstract of the study noted.

More than 43 million people in the United States have no health insurance.

But even people who had health insurance through their employer were among those forced to declare bankruptcy in an effort to escape overwhelming debts that they could not pay, the study found.

About 75 per cent of those who said medical bills triggered their bankruptcy had insurance coverage at the beginning of their illness.

In 38 per cent of the cases studied, that insurance lapsed while they were still being treated, or covered only catastrophic illnesses and not more minor but ongoing conditions.

Among the people whose medical bills contributed to their bankruptcy, the study said, "out-of-pocket costs average[d] $11,854 [US] since the start of illness."

The study's authors said that when the national numbers were crunched, and the dependents of the filers were included, between 1.9 million and 2.2 million Americans experienced "medical bankruptcy" in 2001. That figure includes 700,000 children.