In a matter of days, the U.S. Supreme Court will likely issue a decision affecting nearly one-fifth of the nation's economy. Here's what you need to know.

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Medical care accounts for 18 percent of the U.S. gross domestic product--and it is taking up at least as much of the mindshare of plenty of American entrepreneurs and business owners.

At this very moment, the economic basis of the entire health-care system is up in the air, as a result ofthe imminent U.S. Supreme Court opinion in the health care reform law cases. Court observers expect the decision any day now--perhaps as early as Thursday morning--and most likely before the end of June.

"This is going to be one of the biggest decisions to come down in our lifetime," said Robert Litan, vice president for research and policy at the Kauffman Foundation. "The economic impact could be tremendous."

For American entrepreneurs and employees so far, the waiting has been the hardest part. That's because the decision is likely to have so many ramifications and third-order effects that it's nearly impossible for business owners to forecast all the possible outcomes.

"As the person responsible for setting up health care for our 70-employee company, what I want most is some certainty," said Josh King, vice president of business development and general counsel at Avvo.com, a company for which health care is the second-largest expense after salaries.

"While the [law] has its issues, at least it's a step toward more predictability in health care,"King said. "If the court starts tearing it apart, I fear we'll have to spend more time thinking about health care and less trying to run and grow our business."

Here's how most observers expect the decision to play out--and what that means for this country's small and fast-growing companies.

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Inside the Supreme Court Health-Care Decision