A one-time chief executive of the New York Stock Exchange, Richard Grasso, must forfeit a portion of his $139.5 million pay package, a state judge ordered yesterday after finding that Mr. Grasso failed in his duty to inform NYSE executives of how many millions his pension plan was worth.

The decision by the judge, Charles Ramos of state Supreme Court in Manhattan, found that Mr. Grasso’s reticence on how much his compensation package was worth “thwarted” an executive board at the NYSE “from performing its duty of care and obedience.”

“Mr. Grasso’s duty is to be fully informed and to see to it that the Board was fully informed,” Judge Ramos wrote. “He failed in this duty.”

But the 72-page decision does not address the main allegation Attorney General Eliot Spitzer made when he sued Mr. Grasso: that the amount of compensation was unreasonable for a nonprofit entity. The question of reasonableness, Judge Ramos ruled, must wait for trial.

Nonetheless, the decision represents a victory for Mr. Spitzer in the final weeks before the general election. Mr. Spitzer, a Democrat, seeks the governorship. His Republican opponent, John Faso, has characterized Mr. Spitzer’s case against Mr. Grasso as “misguided” and “phony.”

In his decision, Judge Ramos validates Mr. Spitzer’s decision to go after Mr. Grasso — a decision the attorney general’s critics say exceeds the traditional scope of the office of attorney general. But Judge Ramos found that Mr. Spitzer is acting within his legal bounds.

“Here, the Attorney General represents the investing community all of which rely on the integrity of the market,” Judge Ramos wrote, finding that Mr. Spitzer did not lose standing when the stock exchange became a for-profit institution last year.

“Therefore, Mr. Grasso’s contention that only the shareholders of the NYSE LLC benefit from this action is myopic. The investing public benefits as well.”

Judge Ramos also rejects out-ofhand a defense often used by Mr. Grasso and other board members: that they simply did not know the value of Mr. Grasso’s executive pension plan as it ballooned.

That defense, Judge Ramos said, is “shocking.”

“That a fiduciary of any institution, profit or not-for-profit, could honestly admit that he was unaware of a liability of over $100 million … is a clear violation of the duty of care,” Judge Ramos wrote.

Mr. Grasso, who began working as a clerk at the exchange in 1968, has defended his pay and any denied any breach of duty. Mr. Grasso left the top job at the exchange in 2003 after his substantial earnings package became widely known.

In a statement sent via e-mail, Mr. Grasso promised an appeal and said the decision “is riddled with errors.” Mr. Grasso has maintained that NYSE executives approved the package every step of the way.

Mr. Grasso, in his statement, said: “Justice Ramos somehow rejected the testimony of dozens of directors that they approved every dime they paid me, and decided that these men and women did not know what they were doing.”

The decision does not order Mr. Grasso immediately to cut the stock exchange a check, nor does it make clear how much money he will have to surrender. Both sides are certain to disagree on such amounts in the wake of the decision.

Judge Ramos wrote in the final page of the decision that a hearing is needed to decide the reasonableness of Mr. Grasso’s pay package, which suggests that Judge Ramos could find Mr. Grasso is entitled to a core amount of his executive pension plan.

A spokesman for Mr. Spitzer’s office, Darren Dopp, said in a statement sent via e-mail that the ruling meant Mr. Grasso must return “approximately $100 million” — including the entirety of the executive pension benefits he received.