According to a recent report, 31 million adults were considered underinsured in 2014

...because they had health insurance for a full year but had high deductibles or out-of-pocket expenses relative to their income. Of these:

• 36% of underinsured adults reported expensive medical bills under their current health plans that were not covered by insurance
• 37% said their doctor had charged them more than the insurance would pay and they had to pay the difference
• 25% said their health plan denied payment for medical care they had received
• 20% said a doctor’s office told them they did not accept their insurance

Source: "31 Million People Were Underinsured in 2014; Many Skipped Needed Health Care and Depleted Savings to Pay Medical Bills," The Commonwealth Fund News Release, May 20, 2015, http://www.commonwealthfund.org/publications/press-releases/2015/may/underinsurance-brief-release