Leslie is thundering across the stage, back and forth. It is just a little raised platform, not a real stage, but it is hollow and it makes a wonderfully satisfying echo as she stomps her little white shoes on the suffering boards.

Leslie is not quite 2 years old. She weighs about 25 pounds, but she flings herself down on the stage in the manner of a 200-pound diva in the last moments of "La Traviata." Leslie seems indestructible. One's concern is for the stage.

It wasn't always that way. When Leslie was a month old, she weighed 1 pound, 14 ounces.

The stage that Leslie Ferguson is attacking is at the far end of the "Cafetorium" of Fairfax Hospital, where it looks like a birthday party is going on.

But it's really more of a class reunion, a reunion of babies who lived in neighboring incubators for weeks or months in Fairfax Hospital's neonatology department -- and a reunion of parents who shared the anxious days as they leaned over those same incubators together.

Some 80 youngsters are at the party altogether, ranging in age from a few months to almost 8 years, and they are doing expected birthday party things: clowning, crying, eating, fighting, whining, eating, kicking, grabbing balloons, running, spilling, eating, screaming, laughing, all in that unmistakable level of the child's cacophonous intensity of life.

Most of these boisterous youngsters were under 3 pounds when they were born, some of them under 2. Or they had some of the other massive problems that can afflict newborn babies -- circulatory problems, breathing problems, intestinal problems, misplaced organs. . . .

One thing, though, that put them one up on what used to be gloomy survival odds was that they were born at Fairfax, where neonatology chief Dr. Lloyd Kramer has, in the past nine years, built an operation recognized internationally for its use of the most heroic and innovative methods to save the lives of threatened infants.

Thursday afternoon's reunion, the department's first, required some persistent detective work on the part of hospital staffers and parents of some of the "preemies" in tracking down the 2,500 or so long-term ex-occupants of the smaller-than-doll-size incubators at the hospital. In fact, invitations were issued to about 200 families still in the Washington area, and nearly half came, unanimously and often emotionally singing the praises of the hospital, the nurses, the interns and residents -- at least one child was named for one of them -- and, most of all, for Dr. Kramer.

Kramer stood there and, as they came up to him, greeted them, reading their names from the colorful namecards, perhaps, but remembering the cases even though each parent, one after another, began with "Oh, Dr. Kramer, I'm sure you don't remember me, but . . ."

More than one parent turned away with a healthy brute tucked under an arm and eyes filled with tears. "You know," said one mother, "I get choked up everytime I see him. If it weren't for him . . ." -- and a child is hugged a little closer.

"This is the good part," sighs Kramer.

"Elizabeth just doesn't like waterbeds," her mother, Mary Carol Madej, is saying. "Whenever I put her on one, she just crawls right off."

Elizabeth Moore (her mother kept her own name) is 16 months old, a busy and elusive toddler. She was taken by Caesarean section from her mother at 29 weeks because physicians were unable to control Madej's runaway blood pressure and toxemia symptoms.

Madej and her husband, Garry Moore, a nuclear physicist, were speculating on whether or not the preemie waterbeds, developed by Kramer and his staff -- on which the infants can be rocked and even soothed by music -- had contributed to Elizabeth's current dislike of the genre. She was a shade over 2 pounds after vascular surgery, and spent several months in the hospital. "For a long while," said Madej, who is a university administrator by profession, "I felt as though I was playing with a doll. I could maybe take her out of the incubator for three or five minutes, then I had to put her back. And after I went home . . . well, she was a part of your life, and then she wasn't a part of your life . . ."

As Elizabeth snuggled on her lap Madej said, "It was strange. I thought, 'Am I ever going to feel that mother-daughter bonding?"

Carolyn Ferguson is a Mary Kay cosmetician and part-time model. She is tall, big-boned, blond and hearty. The fact that she couldn't carry to term with Leslie was the source of some guilt feelings, she says now. "Here I was, a nice, strong, healthy woman who ought to be producing baby after baby . . . and I never even had a baby shower."

In retrospect, she sees her experience with Leslie as a positive one (as do Madej and Moore) "because it all turned out so well."

She came to the hospital three times a day to spend time with Leslie and had such an overabundance of milk -- which she delivered via milk-pump to the intensive-care nursery -- that the nurses finally pleaded with her not to bring so much because breast milk can only be stored a short while.

"Leslie's hospital bill was 42 pages," her mother says proudly. "Oh," says Elizabeth's mother, "ours was only 25."

Upstairs in Fairfax Hospital from the cake-smeared faces of those 80 children, a new crop of preemies are fighting for their lives, with the help of a skilled and compassionate staff who administer an IV one moment and cuddle and coo to a 3-pounder the next.

Margaret Reinen has been at Fairfax for some 10 years, slightly predating the advent of Kramer and the evolutions of the facility into one of the top centers of its kind in the country.

She takes a just-fed infant from its incubator and clucks. There is a sort of maybe-smile and the infant, Jennifer, throws her arm back, palm outstretched in a sort of a wave. "The international signal of the preemie," Reinen says.

"Preemie power," says another nurse.