If the need to go the emergency room suddenly arises, your first instinct is likely to be a phone call to 911—not to wonder how much an emergency room visit may cost you. And in an emergency, getting the right care, and getting it fast, should be the focus.

“If you’re having a heart attack, you’re not going to ask if the doctor treating you takes your insurance,” says Karen Pollitz, a senior fellow at the Kaiser Family Foundation.

But be aware: That potentially lifesaving trip to the emergency room may have a high price tag attached to it. A Kaiser-New York Times survey of insured and uninsured people who had difficulty paying medical bills found that ER bills accounted for the largest portion of what they owed.

Going to a hospital that takes your insurance may not prevent you from getting unexpected big bills. Here’s why: About two-thirds of emergency room doctors are independent contractors, who may not be in your insurance plan, according to the American College of Emergency Physicians. And any out-of-network doctor or laboratory may later bill you for whatever your insurance doesn’t cover, a practice called balance billing. “There can be a whole cast of providers involved, some you never even meet, who send you a bill,” says Pollitz.

In addition, if the ambulance company that transports you to the emergency room doesn’t take your insurance, your out-of-network costs could be over $2,000, depending on factors such as where you live and the level of care you receive before you get to the hospital. A breathing tube, for example, may bump up the price. And if your insurer doesn’t consider the ambulance ride essential, you may be on the hook for even more money

You can’t completely protect yourself from these kinds of surprising medical bills, and you never want to put cost concerns ahead of getting care during an emergency.