Forty-two percent of low-income, privately insured, working-age Americans with chronic conditions, such as diabetes or asthma, spent more than 5 percent of their incomes on out-of-pocket medical costs in 2003, up from 28 percent 2001, according to the study.

"The study provides strong evidence that rising health costs and increased patient cost sharing are taking a heavy financial toll on low-income, chronically ill people--even if they have private health insurance," says Paul B. Ginsburg, president of HSC, a research organization funded principally by the Robert Wood Johnson Foundation.

The study also found that more than one in three privately insured, chronically ill people with low incomes--defined as income below 200 percent of the federal poverty level, or $36,800 for a family of four in 2003--lived in families reporting problems paying medical bills.

Nearly half of the 6.6 million uninsured, chronically ill Americans reported medical bill problems in 2003. Among the 3 million uninsured, chronically ill people with medical bill problems, four in 10 went without needed care, two in three put off care and seven in 10 did not fill a prescription in the past year because of cost concerns.

Across the entire working-age population in 2003, 12 percent reported family out-of-pocket health costs exceeding 5 percent of family income, up from 10 percent in 2001.