"Many practitioners had hoped that the Court would use the opportunity in Hughes to clarify the requirements for a plaintiff in an ERISA fiduciary duty case to survive a motion to dismiss," Brian M. Pinheiro and G'Nece Jones, attorneys at Ballard Spahr wrote.

A district court and the U.S. Court of Appeals for the 7th Circuit said that some of the options that the university offered were designed to meet the university's responsibility for prudence and care. The Supreme Court rejected that argument, saying that the 7th Circuit should not have solely relied on the number of  investment options offered by the university. It remanded the case for additional review.

"We're pleased that the Supreme Court has unanimously ruled in favor of employees and retirees in the Northwestern plan, as they previously did in our Tibble v. Edison case, the only other excessive fee case they have taken," said Jerry Schlichter, managing partner of Schlichter Bogard & Denton, attorneys for the plaintiffs. "With two unanimous decisions, the Supreme Court has made clear that the fiduciary duty for plan sponsors is a serious one, and requires that each fund must be monitored, and removed if imprudent."

Schlichter added, "contrary to Northwestern's contention, fiduciaries don't get a pass on having unreasonably expensive or imprudent investments in a plan just because some others are prudent. Plan sponsors also don't get a pass on monitoring each fund by simply having a large number of fund options."

"It's a win for the plaintiffs," said Michael Weddell, director, retirement at Willis Towers Watson.  He said that the ruling was not unexpected, adding that more than 40 similar cases have been filed.

Many of those cases are being settled, Weddell said, noting that such cases are profitable. "They're being driven more by the law firms than the plaintiffs," he added, saying that some plaintiff's firms advertise for clients in these cases.

In a new report, the Congressional Research Service suggested that Congress might want to step in and clarify the issues. For instance, the Supreme Court did not decide whether the disputed retail-class investments in the retirement plans were suitable for participants. And the court did not establish criteria for making that determination, CRS said.

The CRS said that Congress could define the scope of fiduciary obligations in legislation or choose to address the burden of proof that a plaintiff must meet for bringing a viable claim related to excessive fees.

Weddell said that he does not believe that Congress will step in, since there is no huge constituency clamoring for legislation. He added that it might be more likely that the Labor Department steps in and clarifies some issues.

The case already is having some impact. A Georgia federal district court declined to dismiss a 401(k) lawsuit filed against the Columbus Regional Healthcare System.

In declining to dismiss the case, U.S. District Judge Clay Land of the Middle District of Georgia said he had been "awaiting the Supreme Court's decision" before ruling on the motion. "That decision was recently issued, and the pending motion is now ripe for resolution," he added.