PITTSBURGH (AP) _ Surgeons called the unexpected death of the nation's youngest heart-lung recipient, a 12-year-old girl, one of the most dramatic failures ever in organ transplantation.

''The surgeons involved were quite shocked,'' said Dick Riebling, a spokesman for Children's Hospital of Pittsburgh. ''They thought things were sailing along at a great rate.''

Surgeons had been ''very pleased'' with Mary Grantz's progress since her surgery Wednesday morning, Riebling said. On Thursday, Mary's father said he was happy with his daughter's progress and that she looked healthy and was communicating to them by nodding.

''And then there was this dramatic turnaround,'' Riebling said. He said the sixth-grader from Farrell died ''when her transplanted heart ceased to function.''

Riebling said the surgeons, Dr. Bartley Griffith and Dr. Robert Hardesty, had previously performed 24 such double transplants, all at neighboring Presbyterian-University of Pittsburgh Hospital. Mary's was the first heart- lung transplant performed at Children's Hospital.

''They feel this was the most dramatic failure in the transplanted heart- lung (program) that they've ever seen,'' Riebling said.

Riebling said further details of Miss Grantz's death were being withheld at the family's request.

During an eight-hour operation, Mary received organs from a 9-year-old Dallas, Texas, boy who died after falling from a motorcycle. The donor was not further identified.

Mary suffered from primary pulmonary hypertension, a disease that causes high pressure in the lungs and progressive damage to the heart and the lungs.

Her death leaves Elizabeth Burns, 14, of Norman, Okla., the youngest survivor of the complex surgery in the United States. She received a heart and lungs at Presbyterian-University Hospital on June 12 from 11-year-old Shanton Simper of Clarkston, Mich., who died when a blood vessel burst in his brain.

A team of surgeons headed by Griffith performed Elizabeth's operation.

The teen-ager was discharged from the hospital Thursday to be treated as an outpatient, said hospital spokesman Tom Chakurda.

''Her recovery has just been remarkable,'' Chakurda said.

The youngest person ever to receive a heart-lung transplant was a 5-year- old Australian girl, Brooke Matthews, who underwent surgery in London's Hempfield Hospital on March 4, Chakurda said. She has left the hospital and is well, the hospital said Friday.