‘Going private” is all the rage these days for public companies. Now, investors are ready to join them. Scheduled to open this morning will be Portal, a completely private stock market owned by NASDAQ. But unlike traditional brokerage or retirement accounts which can be opened for a couple hundred dollars, Portal has a much higher account minimum: $100 million or more in assets, to be precise.

Portal will open with 500 companies on its register and will allow superwealthy investors and institutions to buy stock just like they would in the public markets. These investors will be able to log onto a secure Web site, obtain data about the company, and with a few clicks of the mouse trade its stock in real-time.

Portal is not alone. On Tuesday, a consortium of major banks including Citigroup, Lehman Brothers, Merrill Lynch, Morgan Stanley, and Bank of New York Mellon, announced that they were setting up a new platform for privately traded stocks. Their system is expected to open in September. One of their big banking brethren — Goldman Sachs — already opened a similar trading market in May.

Because of their asset holdings, these private stock market investors are classified as Qualified Institutional Buyers — “QIBS” — under Rule 144A of the Securities and Exchange Commission. This means that private companies can sell the investors shares and escape regulatory oversight, such as Sarbanes-Oxley. Companies must still provide financial information to these investors and it is left up to investors to decide if the company is being forthright in their data.

These private stock markets follow the exodus of public companies into the private sector. Household names such as Hilton Hotels and Sallie Mae could become privately owned by the end of this year. In a telling sign of just how popular it is to “go private,” the Washington Post reports that last year $162 billion were raised through private stock, compared with $154 billion in initial public offerings.

We find it hard to blame these companies or investors for their desire to seek alternative investments, especially because of Sarbanes-Oxley and its overly burdensome regulations. But if private stock markets do attract a following, the real losers could be ordinary investors. Average people can only invest in these private markets if their mutual fund creates an account to trade. Otherwise, they are shut out because of the asset qualification. If more and more institutional investors begin to withdraw from the regular stock trading, this could undermine those markets in the long-run, hurting both those investors and remaining companies. It might all seem too incredible to believe, but few Wall Street sages predicted 10 years ago that the private financing boom would develop in the way that it has.

This is why Congress would be wise to revisit Sarbanes-Oxley. The section of securities regulations pertaining to QIBS has been in existence since 1990, but only recently have financial firms started these private markets. We do not think the passage of Sarbanes-Oxley and its effects have been purely a coincidence. Rather, it’s clear that the law has already produced these unintended consequences.