One morning last summer, teen-ager Danielle Lynnette Butler got the phone call she had been waiting for. Her doctors at the University of Maryland hospital had found her a kidney!

Danielle, who turned 15 in January, rushed downtown to the hospital with her family. "I had some tests, and then I had the operation," she remembers. "I got the phone call at nine, and I got the kidney transplant at five."

Seven days later, Danielle and her new kidney -- which she has nicknamed Daniel -- were back home again.

Two months later, Danielle was back in her ninth-grade classroom. Dr. Charles Medani, a pediatric nephrologist, or doctor who specializes in kidney disorders in children, is Danielle's doctor. "I met Danielle when she was 9. Her kidneys were beginning to fail."

Danielle's kidney disease meant that she had to undergo dialysis -- a procedure that washes the blood in a machine -- several times a week. Danielle was on dialysis for years, until that day a year ago when she had her transplant operation.

Today, a year later, Danielle says she is "feeling fine."

When illness or injury makes the kidneys shut down, Dr. Medani explains, the human body slowly begins poisoning itself. That's because the kidneys' job is to remove waste products from the blood. When the kidneys stop working well, people become weak and sick. Children with kidney disease may not grow to a normal height.

The poisons left in the body from kidney disease can even cause a person's personality to change. Left untreated, kidney disease finally causes death.

Kidney disease is much more common in adults than it is in children. Experts in nephrology -- the branch of medicine concerned with kidney disease -- fight the illness with drugs, with dialysis and with kidney transplants. Just as a plant in the garden can be moved -- or transplanted -- from one spot to another, a kidney can be moved from one body into another one.

A kidney transplant is a serious and difficult operation, and it sometimes fails. But when it works, as it did for Danielle, the results seem almost miraculous.

"After a transplant, we may see normal blood tests as soon as two or three days later," says Dr. Medani. The normal blood tests show that the new kidney is successfully removing poisons from the blood.

The human body comes equipped with two kidneys. But just one is enough to keep the blood clean. Some kidneys used in transplants come from relatives whose tissues "match" the patient's.

"If there isn't anyone in the family who can donate a kidney, you wait until someone who is normally in good health has died from an auto accident or a head injury. If the kidney is intact, and if it matches, some families will allow doctors to take the kidney to help a sick person get better."

Doctors test tissue and blood from the donor to see if the organ will be accepted by the patient's body. Sometimes, the patient's body rejects the new kidney.

New drugs that have been tried in recent years have made rejection less likely. One drug called cyclosporin has been especially useful. Since it was introduced a few years ago, the number of patients who reject their new kidneys has gone down.

Danielle takes cyclosporin to help her body adjust to her new kidney. After a kidney transplant, Dr. Medani explains, the patient can go on to lead a normal life. "They can go to school, drive a car, go swimming," he says. "But it is very important to avoid doing things that would cause bumps."

The kidneys someone is born with are well-protected from bumps and bruises. They're located deep inside the body and are guarded from harm by the hard bones of the rib cage. But the kidney a patient receives as a transplant is placed in the abdominal cavity -- a hollow area below the bellybutton, close to the surface. That's why avoiding bumps is so important.

Since her operation, Danielle has kept up the normal activities you'd expect from a ninth grader. She didn't take gym last year but thinks she might be able to this fall. "I have more energy now," says Danielle. "I get to do more things now. I met more people at school, and I got more work done because I was at school more."

This summer, Danielle is enjoying swimming and trying to find a job. She's not too sure what kind of summer job she'd like. But she's sure about what kind of work she wants to do when she grows up.

"I want to be a doctor," she says. "I want to do nephrology. I think I could help because I've been through it myself."Tips for Parents

After a 2 1/2-year wait, Danielle Butler's family finally found her kidney through a regional list of donor kidneys in the Baltimore area. In the Washington area, the largest mid-Atlantic transplantation service is administered at the Washington Hospital Center in the District. For more information, call 541-7777. For information about donor cards and about kidney disease in general, contact the National Kidney Foundation of the National Capital Area, 2233 Wisconsin Ave. NW, Wash., D.C. 20007; phone 337-6600. Catherine O'Neill is a free-lance children's writer based in Baltimore.