A new study published in medical journal JAMA Monday shows that Americans are spending more out of pocket for major health care expenses. The study, lead by Emily Adrion of the University of Michigan Medical School, analyzed claims for 7.3 million hospitalizations between 2009 and 2013. In that time, the total amount paid by patients—the vast majority of whom had employer-sponsored health insurance—grew by more than 37%, the researchers found.

While health care spending increased overall—2.9% per year during the study period—the amount shouldered by patients grew more than twice as quickly, at 6.5% each year. In real terms, the average amount a patient spent grew from $738 in 2009 to more than $1,000 in 2013. The data used in the study was from insurance companies Aetna (AET), Humana (HUM), and UnitedHealth (UNH), which collectively have about 50 million users.

“There have been so many stories about how much hospital costs are for the uninsured and underinsured, but what’s lost is how much people with insurance are paying,” explains Adrion. The $1,000 price tag for a hospital visit was “higher than we would have thought,” she says.

The reason for the increased spending was driven primarily by the increase in the amount applied to deductibles, which rose by 86%, according to the study. Coinsurance also increased by 33% during the study period.

There seems to be a trend toward plans with higher deductibles and coinsurance and lower copayments, though it is unclear whether this is due to a shift in patient choices or employer offerings, says Adrion, though “it’s definitely worth looking into.”

Furthermore, because the vast majority of enrollees were in employer-sponsored plans, “these trends are likely to continue,” she says.