Seventy-two million working-age adults -- about 41 percent of the population -- are having trouble paying off medical debt, according to a new study from the Commonwealth Fund.

The group said 34 percent were having problems in 2005.

It also found that 7 million people over the age of 65 are in trouble, for a total of 79 million people reporting problems.

In 2007, nearly two-thirds adults -- 116 million people -- had medical bill problems or debt, went without needed care because of cost, were uninsured for a time or were underinsured.

The group said the problems come from rising costs, people losing coverage through work and higher out-of-pocket costs.

"We are seeing a perfect storm of negative economic trends threatening working families in the United States," said lead author Sara Collins.

The report said that low- and moderate-income families face the most problems. More than half of adults earning less than $40,000 a year reported problems paying bills.

More than half of insured working-age adults who said they have deductibles that represent 5 percent or more of their income reported medical bill burdens and debt; one-third of adults with lower deductibles face these kinds of difficulties.

People also reported that concern about costs can lead to skipping doses, leaving prescriptions unfilled or not visiting the doctor.