A 26-hour effort to separate a pair of German twins who were joined at the head resulted in the death of one of the girls early yesterday morning at Johns Hopkins Children's Center in Baltimore, hospital officials said.

The surviving sister was in critical but stable condition, officials said.

Doctors spoke grimly yesterday of the surgery to separate Lea and Tabea Block, 13-month-old conjoined twins from Lemgo, Germany, with bright blue eyes and boisterous smiles.

"I speak for the entire team in saying we are deeply saddened by Tabea's passing," said Benjamin Carson, director of pediatric neurosurgery at the center, adding, "We have high hopes for Lea, who now has a good chance at leading a healthy, independent life."

The delicate operation had begun Saturday but was halted after Tabea's heart stopped twice during eight hours of surgery. Doctors kept the girls under anesthesia while they recuperated. A team of 95 doctors, nurses and technicians resumed work early Wednesday to tease apart the girls' brain tissue and separate their blood vessels.

But shortly after the operation concluded yesterday at 12:15 a.m., Tabea Block died after her blood pressure dropped precipitously and could not be restored. Lea remains in the hospital's intensive care unit. She is scheduled for additional surgery next week to cover the skull with her stretched skin.

Conjoined twins occur only about once in every 200,000 live births, and the survival rate is 5 to 25 percent. Doctors said that twins who are joined at the head -- which occurs once in every 2 million live births -- are much more difficult to separate than other types of conjoined twins. In June, surgeons at Children's Hospital in the District successfully separated Jade and Erin Buckles, who were connected by a liver and a small bridge of tissue across their hearts.

That same month, Lea and Tabea came to Johns Hopkins to begin preparations for their surgery. Tissue expanders were inserted underneath their scalp and filled with a sterile solution, causing the skin to swell into large bulbs. The hope was that each twin would have enough skin to cover her head after the surgery.

The twins' parents, Nelly, 27, and Peter, 28, have signed an exclusivity agreement with the German magazine Stern and have declined to be interviewed or photographed. However, pictures of the family on the magazine's Web site show the twins dressed in matching outfits, taking baths in an extra-long tub and playing on a large blue exercise ball.

In an interview with the magazine printed last week, the Blocks said that the girls have developed as normally as possible: They could say "momma" and "poppa." Their first teeth were poking out. They could crawl on all fours, although their head never came off the ground.

Tabea apparently was born with heart abnormalities that became apparent during diagnostic procedures the twins underwent before surgery and turned increasingly serious during the operation, doctors said.

When the operation was halted Saturday after eight hours, the physicians feared that the pumping ability of Tabea's heart might have been permanently damaged. However, after several days in intensive care, her cardiac function returned nearly to normal, and the team decided to resume the operation.

Late Wednesday night, when the surgeons were 90 percent finished separating the infants' brains and dividing the complicated network of veins, Tabea developed first a rapid and then an unusually slow heart rhythm.

"We had to go into sort of an emergency mode to get them separated," Carson said. "It was clear that we did not have much time."

The last large blood vessels, called venous sinuses, were divided in more than an hour -- work that would normally have been taken two to three hours.

Some of the vessels had to be temporarily clipped together rather than sewn. There was extensive bleeding at this point, although more from Lea, the surviving twin, than from her sister, said Allesandro Olivi, one of the neurosurgeons.

Once the girls were separated, part of the team tried for an hour to resuscitate Tabea. But she never regained a normal heart rate or blood pressure. In all, they lost and had replaced through transfusion roughly three times the amount of blood in their tiny bodies, said Deborah Schwengel, a pediatric anesthesiologist.

Asked if he would have done anything differently, Carson said that given another opportunity, he would first place devices that would expand the twins' skulls as well as their scalps.

That, he believes, would have made the unusually complicated procedure somewhat easier.

Benjamin Carson, shown in a 2003 photo, was on the surgery team that worked for 26 hours on the girls, who were joined at the head.