62% of personal bankruptcies in the US are caused by medical costs.

Source: cbsnews.com

Additionally, Bloomberg Businessweek reports:

Medical problems caused 62% of all personal bankruptcies filed in the U.S. in 2007, according to a study by Harvard researchers. And in a finding that surprised even the researchers, 78% of those filers had medical insurance at the start of their illness, including 60.3% who had private coverage, not Medicare or Medicaid. You can read more about it here.