A 19-year-old woman who suffered brain damage and lost her short-term memory after doctors operated on the wrong side of her brain will receive $3 million in a settlement reached yesterday in U.S. District Court here that is one of the largest medical malpractice settlements in D.C. history.

According to court papers, healthy brain tissue was removed from the left side of the brain of Teresa Lewis, then 15, after a technician incorrectly labeled a CAT scan X-ray, reversing the location of a brain tumor.

Lewis and her family will receive $1.5 million from George Washington University Hospital, where the CAT scan was performed, and $1.5 million from Georgetown University Hospital, where the operation took place. The Lewises also agreed to drop their suit against the operating surgeon, Georgetown neurosurgeon Alfred J. Luessenhop.

The agreement, approved by Judge George H. Revercomb, denies any liability on the part of the hospitals.

"I think that our hope is that doctors everywhere will be able to learn of this case and benefit from it so that it will never happen again," said David Lewis, Teresa's father. "I could not even begin to describe what our lives have been like. It placed my daughter in another world . . . .

"We lost a very functionable person who now needs constant care and attention 24 hours a day."

Attorneys for George Washington and Georgetown yesterday declined to comment.

Doctors began to realize there was something wrong with Teresa Lewis' memory the day after she underwent surgery at Georgetown in the fall of 1982 for what doctors believed was a tumor on the left side of her brain. Lewis, they discovered, could not remember things that were told to her 30 seconds earlier. A follow-up CAT scan showed that, in fact, the tumor was on the right side.

Because healthy brain tissue had been removed from one side of the brain while there was a tumor on the other side, brain damage and memory loss occurred, according to depositions given by doctors in preparation for the suit. If the operation and tumor had been confined to the same side of the brain, the injury would not have occurred, the doctors said.

The Lewis' lawyer Jack Olender contended in court papers that George Washington had been negligent in its preparation of the CAT scan.

Georgetown and surgeon Luessenhop, he argued, were negligent because they relied solely on the George Washington CAT scan and removed brain tissue despite its healthy appearance.

Olender said yesterday he planned to place ads in medical journals describing what happened to warn other hospitals.

Lewis underwent subsequent surgery to remove the tumor. Court papers show that her short-term memory is frozen to the time preceding the surgery and that she has great difficulty learning anything new.

Her father said she has no comprehension when she reads and her memory of things told to her now lasts only five to 30 minutes. As a result, he said, she cannot be left alone and doctors told the family she will never be able to function on her own or be gainfully employed.

She is expected to receive a certificate of attendance from her Prince William County high school this year.

"At first she used to cry, but now she just doesn't do much," David Lewis said.

"She watches television occasionally but she has no outside interests other than our church . . . .

"I go to bed all the time dreaming it really didn't happen . . . . It is an unreal prison."