A grand jury investigation of the White House sex scandal took a surprise turn Tuesday when prosecutors abruptly postponed an appearance by Betty Currie, President Clinton's personal secretary.

Sources familiar with the investigation said Starr's office late Monday suddenly canceled Currie's previously scheduled appearance, giving no reason. A prosecution source said no other witnesses had been called to testify Tuesday.Currie, a potentially pivotal witness in independent counsel Kenneth Starr's probe of Clinton's relationship with former White House intern Monica Lewinsky, told reporters outside her suburban Virginia home she would be going to a local hospital and then on to her job at the White House.

She gave no reason for her hospital visit.

Currie was the first witness summoned in late January when Starr's grand jury began looking into whether Clinton had an affair with Lewinsky and illegally conspired to cover it up by inducing the young woman to lie under oath.

News pictures of her cowering before a swarm of reporters and photographers as she left the federal courthouse after testifying Jan. 27 helped contribute to a negative public perception of Starr's investigation.