http://www.ama-assn.org/ama1/pub/upload/mm/368/reportcard.pdf (big pdf file!)
The chart below appears on page 5 of the 16-page report.
Of the eight insurers listed, Medicare is most likely to reject a claim, sending away 6.85% of requests. This is more than any private insurer and double that of the private insurers’ average.
http://biggovernment.com/2009/10/05/ama-endorses-largest-denier-of-health-care-claims/