Having health insurance can be a comfort, putting your mind at ease that you’ll be covered if you get sick or injured—until you actually have to use it, that is.

Insured Americans are having to shell out more and more for healthcare, particularly, hospital visits, researchers report this week in the journal JAMA Internal Medicine. From 2009 and 2013—before the biggest provisions of the Affordable Care Act took effect in 2014—people with individual or employer-sponsored health insurances saw a 37 percent rise in out-of-pockets costs for a hospital stay. Average bills jumped from $738 to $1,013. That’s about a 6.5 percent increase each year. However, overall healthcare spending rose just 2.9 percent each year during that time-frame and premiums—the cost to buy insurance—rose by around 5.1 percent annually.

“Every year, people freak out about how high premiums have gotten and how they continue to grow exponentially, but [out-of-pocket costs are] actually growing even faster,” Emily Adrion, first author of the study and a researcher at the Center for Healthcare Outcomes and Policy at the University of Michigan, told Bloomberg.

In the study, Adrion and colleagues looked at data from more than 50 million people insured by one of three insurance giants: Aetna, Humana and United Healthcare. In addition to hospital stays in general, the researchers also looked at specific reasons behind visits. They found that the out-of-pocket bill for a stay due to an appendicitis jumped 40 percent, from 1,079 to $1,509. Costs of heart-attack related stays rose 37 percent, from $1,158 to $1,586.

Those fattened bills linked to fees that most people might not be very familiar with. For instance, the amount that hospitalized people paid toward their deductible, a sum that individual shell out before insurance kicks in, rose by 86 percent during the five-year period. And costs for coinsurance, the percentage of the hospital bill individuals have to pay themselves, rose by 33 percent. A 2013 study in the Journal of Health Economics, found that of 202 people surveyed only 14 percent had a firm understanding of coinsurance, deductibles, co-payments, and maximum out-of-pocket costs.

Adrion told Bloomberg that she expected that these trends are continuing, leaving many insured Americans to face financial stress if they fall ill.

JAMA Internal Medicine, 2016. DOI: 10.1001/jamainternmed.2016.3663