Today's New York times has an interesting essay that describes how a woman reviewed her own medical file and secretly removed all references to the fact that she has a family history of Huntington's disease. She did so out of fear that the information, in the hands of insurance companies, might prejudice her children and their ability to get coverage.
It's an interesting illustration that not only are some patients not being forthright with their physicians out of fear where their information may end up, but some will take more drastic action. Both can have a serious impact upon the health care they receive. See: The Quest for Privacy Can Make Us Thieves - New York Times.