Sources of income, any use of off-shore bank accounts and the overall health of his various businesses would likely be revealed, according to two legal experts.
"In terms of what could be in them -- it's anybody's guess," said political law expert Kenneth Gross. "There could be no problems. There could be problems on the amount of tax he's paying, deductions he's taken, income he's taken, any host of issues in the calculation of tax."
Prospect of Paying a Low Tax Rate
Romney said in a Facebook post on Wednesday: "It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters, especially one who has not been subject to public scrutiny in either military or public service."
For Trump's part, he's made it clear that he has tried to pay as little as possible.
Under Trump's proposed tax plan, anyone making more than $150,001 would have a 25 percent income tax rate.
When asked on "GMA" to share his own tax rate, Trump said: "It's none of your business."
Historical Precedent of Releasing Tax Returns
Assessing Trump's Wealth and Business Assets
There has been much speculation about Trump's wealth since his self-estimates place his net worth at $10 billion. Other outside groups, like Forbes, put his net worth at $4.5 billion.
Even if the tax returns are released, that doesn't mean the question of his true net worth will be solved, according to Rachel Stockman, an editor at LawNewz.
"He wouldn't be required to report [the actual value of his assets]. He would just have to report the money coming in and coming out of his businesses and whatever else he owns, including investments," Stockman told ABC News.
"It would give us a good indication, however, of the financial conditions of his various businesses," she said.
Stockman said she spoke to about half-a-dozen tax attorneys and only one said that she would advise her client to release their tax returns while under audit.
"The concern from tax attorneys was that public opinion could influence the auditors," Stockman said.