They probably will be able to immigrate. Medical conditions are only a basis to deny admission in permanent resident status if a person has a communicable disease or some disorder that has posed a threat to themselves, others, or property. The other issue raised is whether your parents are a likely public charge. As a petitioning relative, you are required to file an affidavit of support to show that you could support them at 125% of the poverty level. If there are unusually high medical costs, you might need to prove that you could provide for those. For example, you might need to prove that you could afford to by medical insurance for them.
Mr. Crabtree's answer is concise and on-point.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
If you're seeking to bring them to the U.S. for the remainder of their days, the process is very straightforward and you should be able to get them here within a year. The issue of the medical condition is two fold. If it falls under a disease of public significance, then you'll need a waiver for it. If it requires significant assets for the medical treatment, you'll simply have to show that they can obtain appropriate insurance.