The former Massachusetts governor's individual retirement account is estimated to be worth $20.7 to $101.6 million, and estate planning experts say it is highly unusual for someone to accumulate so much money in an IRA, which is restricted by annual contribution caps.
Romney's IRA grew so large because it holds investments in Bain Capital LLC, the private-equity firm Romney helped found, The Wall Street Journal reported Wednesday.
IRA experts said Romney isn't legally required to pay annual taxes on the account's investment gains and most of his annual contributions to the fund are likely to have been pretax dollars, which allows him to defer paying taxes on a substantial part of his fortune.
However, Romney may have to pay as much as 35 percent tax when he withdraws from the fund, experts say.
"It's probably not a slam dunk," said Jonathan Rikoon, a lawyer at New York's Debevoise & Plimpton LLP.
Romney said this week he would probably release his tax return sometime around April, following pressure from other presidential candidates. He said his effective federal income tax rate had been about 15 percent.
"I can tell you we follow the tax laws, and if there's an opportunity to save taxes, we, like anybody else in this country, will follow that opportunity," he said last month.