The first patient will leave Children's Hospital about 7 a.m., at the start of what everyone hopes will be a calm, spring-like Saturday in May.

In three-minute intervals, 90 or so children in acute care will be put on gurneys and directed to one of two elevator banks for the ride to the ground floor. Another 60 more seriously ill patients will have more equipment and will need more time.

Over the ensuing 10 hours, the Children's Hospital staff intends to seamlessly transport these young patients a distance of 2 1/2 miles to the new Children's in Lawrenceville.

On paper, it sounds easy -- a simple 10 to 12 minute drive from the facility in Oakland, up Bigelow Boulevard and over the Bloomfield Bridge, passing through six to eight traffic lights before arriving at the new campus on Penn Avenue.

"We're not driving to the Rocky Mountains or anything; we're just driving across town," said Eric Hess, the Children's vice president who shoulders primary responsibility for the May 2 move.

"But it's definitely not simple."

Consider this: Mr. Hess and Patient Move Coordinator Jennifer Iagnemma won't know until hours beforehand exactly how many children they have to move, or the precise medical status of those children.

Some will be on ventilators or a treatment known as extracorporeal membrane oxygenation, which requires major technical support and heavy machinery.

Some will be newborns, small enough to be held in one hand, who must stay in temperature-controlled isolets at all times.

The patients will be classified as Red Track or Blue Track, the first for the critically ill coming from intensive care, the second for the acutely ill. Fourteen ambulances will handle the most seriously ill patients and 20 other ambulances -- some on loan from outlying counties -- will handle the rest.

"We don't want to have any moment when we have a patient without an ambulance," said Mr. Hess. And, because small children are involved, they have to accommodate at least one parent on each trip.

The doctors, nurses, aides and technicians at Children's are accustomed to moving fragile children every day, in all kinds of situations.

But "you don't do them all at once and continuously for hours," said Barb Joers, the chief operating officer at Monroe Carell Jr. Children's Hospital at Vanderbilt in Nashville, Tenn., who oversaw the transfer of 113 children to its new hospital in 2004.

By comparison, she had it easy -- the Vanderbilt move was less than 1,000 yards, all through indoor corridors and tunnels. But the route took them by labs and other areas not set up for patient care, so they had to set up crash carts along the way in case of an emergency. They were done in five hours.

Meanwhile, Denver Children's move one year ago involved transporting 111 patients eight miles from Downtown Denver to a new facility in Aurora, Colo. "We're dealing with kids who are literally maybe hours or days from birth, all the way up to their teens," said Jerrod Milton, who directed the move.

They got help from ground transport teams and equipment at Children's hospitals in Little Rock, Ark.; Kansas City, Mo.; and Columbus, Ohio. That meant making sure clinical staff from out of state met Colorado licensure requirements. And because part of the route was on a federal highway, they needed to coordinate with state police as well as city police at both ends. "We had a lot of different jurisdictions we had to work with."

He described it as "a very controlled process," without lights or sirens as the ambulances moved through town -- and, best of all, without incident. "As an outsider passing by, you might not have picked up that we were moving an entire hospital."

That's what Mr. Hess and Mrs. Iagnemma hope people say after next May's move.

Already, 191 days away, every step of the process has been coordinated: which exits and entrances they'll take, which elevators, which ambulances, what's the backup route, where they will unload once in Lawrenceville.

"I know we can get a patient prepared and on the stretcher, and on the elevator, that they will have enough staff," said Mrs. Iagnemma. "My biggest concern was how do you get them across town?"

On Mr. Hess' worry list is something more mundane: elevators.

Specifically, he has focused on the two banks of four elevators each at the Children's facility in Oakland. The 150 children are on floors seven to 10, and they have to get them out and into an ambulance with minimal delay while the rest of the hospital is still operating.

"If one little person has a mishap," said Mrs. Iagnemma, "we have to keep the move going."

There is one complication already.

The Pittsburgh Marathon will be run on May 3, the day after the patients are moved but while some of the operating room and other medical equipment needs to be moved. The marathon runs down Liberty Avenue, crossing in front of the Bloomfield Bridge and dissecting the route between the old and the new Children's Hospitals.

So, rather than risk a delay, Mr. Hess is sending the trucks down Fifth Avenue to the Boulevard of the Allies toward Downtown, across the Veterans Bridge then back up Route 28. "That kicked us to a different level of planning," he said. "But it's best for us to keep moving rather than to have the haphazardness."

There were the other considerations, too: They needed to make sure the Petersen Events Center was not holding any events that day. They checked with the city to make sure there weren't any road construction or paving jobs scheduled along the route.

Then there are the variables.

"You can't stop a rainstorm from coming through town, or an infrastructure fire along the main route," said Ms. Iagnemma. The secondary route, if needed, takes them nearer the Carnegie Mellon campus, adding another layer of complication to the planning.

Is there a rain date?

"I don't let anybody ask that question," said Mr. Hess. "We need to stay focused and get done what we need to get done."

In a sense, they're like athletes preparing for an Olympics, training and practicing for years for an event they have one chance to get right.

"It keeps me up nights, but it's very exciting," said Mrs. Iagnemma, a nurse who's been at Children's for 20 years. "It's probably something I'll only do once in my life."

Once the Denver move was finished without complication, "It was an exhilarating kind of moment," said Mr. Milton, who added that there was a champagne celebration in the hospital boardroom afterward. "It was an adrenalin-filled day. It was definitely something to come down from."

Ms. Joers agreed. "For that day, you could see the details of how a hospital runs every day, details you take for granted. It's very exciting. I would do it again in a heartbeat."