Surprise medical bills are top of mind for American patients, with 38 percent reporting they were “very worried” about unexpected medical bills.

About 1 in 6 Americans were surprised by a medical bill after treatment in a hospital in 2017 despite having insurance, according to a study published Thursday.

On average, 16 percent of inpatient stays and 18 percent of emergency visits left a patient with at least one out-of-network charge. Most of those came from doctors offering treatment at the hospital, even when the patients chose an in-network hospital, according to researchers from the Kaiser Family Foundation. Its study was based on large employer insurance claims. (Kaiser Health News is an editorially independent program of the foundation.)

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The research also found that when a patient is admitted to the hospital from the emergency room, there's a higher likelihood of an out-of-network charge. As many as 26% of admissions from the emergency room resulted in a surprise medical bill.