The "Two Marias," twin baby girls from Guatemala, were born joined at the head, and then surgically separated in Los Angeles.
Since then, their lives have taken some surprising turns. Their luck has not been equal.
It took Dr. Jorge Lazareff and his surgical team 23 hours to successfully separate Maria de Jesus and Maria Theresa at UCLA. Although it was a risky operation to separate the twins, who were connected at the skull and couldn't even see each other, it was an easy decision because only 15 percent of conjoined twins survive to age 5.
Lazareff remembers the girls' father saying his children must have a chance at living solo.
"You look at them and everybody says how cute they are," Lazareff recalls, "When they will turn 15, everybody will look at them as monsters … and we don't want that."
The enormous cost of the miracle in progress is funded by private donations. Actor Mel Gibson helped start the Two Marias International Children's Fund, which assists not only Josie and Teresita but medical treatments for children from more than two dozen countries.
Maria de Jesus, whose nickname is Josie, and Maria Theresa, who nickname is Teresita, are now 4½ years old. They have learned that even medical miracles don't come easy.
Six months after the surgery, the girls had healed enough to return to Guatemala. But then meningitis attacked Teresita's brain and both girls were returned to California for treatment, where they've been ever since.
Today, Teresita, who before the separation was the stronger, dominant twin, has suffered mightily. Brain damage has left her unable to hear, speak or walk. But Josie flourishes and attends preschool.
"She doesn't miss a thing," said Jennifer Hull, Josie's foster parent. "She loves life and she loves the world and she loves people."
Monica Morales, Josie's preschool teacher, agrees.
"Just to see the smile on her face every morning when she walks through the door, it just warms your heart," she said.
But Josie still has work to do on her medical miracle, too. She is using a walker now, and taking her first steps. Soon she will have neck surgery to help her hold her head up more easily.
The real concern, however, is for her sister, Terecita, whose foster family is trying hard to break through the brain damage.
Flore Cayas, Terecita's foster mother, said the little girl is also starting to enjoy life.
"She babbles," Cayas said. "She moves. She's starting to enjoy life. There is someone in there. There's not a doubt at all."
Because of all the work to be done, the twins remain not only separated from each other but from their family, too. There are phone calls from home in Guatemala and visits several times a year, but being away is hard on the family.
"They are my daughters and they should be with me," said their father, Wenceslao Quiej Lopez. "We've learned to accept it slowly. As long as they are there they will get better medical care."
The twins' parents are expecting a new baby -- a boy -- any day. They plan to name him Jorge after Lazareff, the surgeon who separated the twins.
ABC News' Jim Avila originally reported this story for "World News Tonight."