Are you sure? I will buy into significant, but I doubt that it is the bulk of the costs. I generally feel that doctor fees themselves are usually pretty reasonable, when you consider the amount of staff the average GP has to support, and much of the staff is nonmedical stuff like Medicare compliance, and dealing with all the different medical insurance companies. Hospital charges, on the other hand, seem divorced from reality, though there could be factors I'm not aware of.
I think it is a major contributing factor to the underlying costs. Many doctors practice defensive medicine now (defense against being sued for not doing something). It's not uncommon for laboratory biopsies to be ordered for every scrap of tissue which is removed even if there is no reason to suspect cancer. And it's not uncommon for MRI's and other procedures to be ordered simply because if they are not, and later something turns up, the Doctor can be sued.
No single problem lies at the root of health care costs, but I believe that the interaction between legal concerns (malpractice) and the way the insurance industry manages premiums, exceeds that actual cost of pure health care science (medicine).
And then there's the pharmaceutical industry and the price controls they maintain through political lobbying...
If we could start from scratch and build a health care system which had nothing to do with legal issues or insurance or price controls, then I think an efficient system could be designed and then implemented. Unfortunately, such systems need to be designed by scientists and engineers, not by politicians who are worried about who gives them the largest campaign contributions.