Houston residents voted decisively on Saturday to exempt their city's pension plan from a state requirement that pension promises be kept.

About 73 percent of the ballots cast in the special election were in favor of opting out of the pension requirement, which became part of the Texas Constitution in 2003, according to the clerk for Harris County, which includes Houston. 

Houston put the matter to the voters amid growing concern about the solvency of the city's pension fund. Pension officials added an unusually generous package of benefits to the plan in 2001. The package attracted little notice at the time, but the cost has since climbed.

Similar problems have cropped up in other cities and counties that set up the same type of benefits package in the last few years. The package is usually called a DROP, for deferred retirement optional program.

In Houston, taxpayers learned this year that their city's DROP had left them on the hook for pension benefits far richer than those offered by private-sector companies, or by other cities of comparable size.

An actuarial study commissioned by the city in February showed that hundreds of public employees would qualify for special, one-time payouts of more than $1 million each when they retired and that some public workers stood to earn more as retirees than they did when they worked. 

The study was done by Joseph Esuchanko of the Actuarial Service Company in Troy, Mich., who also calculated that the city pension fund had fallen about $1.5 billion short of the amount it needed to pay all the benefits it owed. He said the shortfall would grow in the next few years.

The Harris County district attorney, Charles A. Rosenthal Jr., has been looking into possible wrongdoing in the sweetening of Houston's pension benefits. Mr. Rosenthal declined to discuss the inquiry before the election, saying he did not want to prejudice the vote. The constitutional amendment gives municipalities in Texas a single opportunity to opt out of the pension requirement, by holding a referendum.

Mayor Bill White has said he does not want to reduce city workers' pensions. But he said Houston needed more flexibility to keep the benefits affordable.