A federal court jury ordered a Washington pediatrician and Children's Hospital yesterday to pay a Great Falls man $5.75 million in the one of the largest medical malpractice suits ever decided here.

The six-member jury awarded the damages to Alan J. Lee, 22, a junior at Catholic University, after 45 days of testimony during which Lee's lawyer argued that between 1969 and 1971 Dr. Beale H. Ong and Children's made a mistaken diagnosis that Lee was suffering from cerebral palsy.

In late 1971, doctors at Johns Hopkins University Hospital found that Lee was suffering from a spinal cord tumor. Lee is now a quadriplegic and requires round-the-clock care, according to court papers and testimony at the trial.

Henry Lloyd, Lee's attorney, said after the verdict that the Lee family was "pleased and gratified by the award. But these are hard decisions and he won't make him whole again."

Ong's attorney, L. Palmer Foret, said last night that the defendants expect to appeal because they believe the verdict was not supported by the evidence.

Another physician who treated Lee, Dr. Gloria Eng, was cleared of liability by the jury. U.S. District Judge John Garrett Penn ruled that the two partners in medical practice with Ong, Drs. Gordon W. Daisley and William A. Howard, would have to bear the costs with him.

According to court papers and testimony, Lee's condition was first diagnosed as cerebral palsy in 1967, when he was 3 and the same diagnosis was made by several doctors. By 1969, Lee's family was living in Fort Wayne, Ind., and he and his mother had enrolled in a support group for families with children who had cerebral palsy.

While in that group, Sylvia Lee noticed that Alan did not have the mental defects common to children with cerebral palsy, and that his coordination and overall physical condition were deteriorating markedly.

When she sought an additional medical opinion on Alan's condition, she was referred to Dr. Richmond Paine, a neurologist at Children's. But by the time Alan Lee arrived at Children's in 1969 Paine had died and Lee was referred to Ong.

Alan Lee visited Children's five times between June 1969 and October 1971 and his condition worsened from being able to walk alone, although unsteadily, to the point where he had little use of his arms and legs.

Although numerous tests were conducted, the testimony showed, he was not given a myelogram, a kind of X-ray picture of the spinal cord.

By mid-1971, doctors at Children's said that Lee's problems were primarily motivational and he was referred to Hopkins' John F. Kennedy Institute for the chronically disabled. Doctors there discovered the tumor, which could not be removed.

Surgical procedures were performed to relieve the pressure on Lee's spinal cord, and he completed radiation treatments, but the nerve damage could not be reversed.

The defense contended that earlier diagnosis would have made little difference in Lee's condition.