The patient, a 66-year-old woman with cancer in her left breast, lay in a hospital in El Paso, Tex. The doctor about to make her diagnosis was nearly 2,000 miles away in Southwest Washington.

Sitting at a computer terminal in the Comsat building in L'Enfant Plaza last Wednesday, Dr. Alexander Miller, an expert in gynecological pathology, studied a televised image of a biopsy, a microscopic sample of tissue taken from the patient's breast. He quickly determined that the tumor had spread throughout the breast and that more surgery was necessary.

Miller relayed his diagnosis to the patient's doctor through a speaker telephone, suggesting further surgery followed up by possible radiation and chemotherapy.

Long-distance consultation is not unusual in this era of specialized medicine and conference calls. What made Miller's diagnosis remarkable was that the tissue sample he based it on was physically more than halfway across the country, under a microscope in Fort William Beaumont Army Medical Center in El Paso.

The so-called telepathology system, demonstrated last week at the Communications Satellite Corp., combines recent advances in satellite communications, high-resolution television, sophisticated microscopy and computer-controlled robotics. Its centerpiece is a motorized microscope attached to a video camera and a computer.

Through simple keyboard commands, the pathologist had complete remote-control of the microscope, adjusting its magnification, focus and illumination as easily as if it were right in front of him. And the color picture, beamed to Miller's television screen by way of a satellite 23,000 miles above the earth, was detailed and clear enough for him to make the diagnosis quickly and unequivocally.

The demonstration -- sponsored by Comsat, which provided the satellite communications, and Corabi International Telemetrics, which designed the computer software for the remote-control microscope -- was based on a real case at the Beaumont Medical Center. The actual diagnosis in the case had been made the day before by pathologists at Beaumont, but Miller, with no previous knowledge of the case, came to the identical diagnosis after examining the biopsy on the remote television screen at Comsat.

Telemedicine, in which doctors practice medicine at a distance from their patients by studying images and information displayed on a television screen, is a growing field. Some have described the concept as a "hospital without walls."

Massachusetts General Hospital set up a pioneering telemedicine link with Boston's Logan Airport in 1968. The system enabled doctors at Mass General to send prescriptions and study blood smears and X-rays for patients at the airport. They could even listen to a patient's heartbeat with an electronic stethoscope.

But last week's demonstration at Comsat was the first time a pathologist had used such a system to make an actual diagnosis from a microscopic tissue sample whose image was transmitted over great distance by satellite communication. Until recently, neither the quality of the video transmission nor the sophistication of the computer software was good enough to produce the fine-detail image needed for a remote pathology diagnosis.

"The technology is now available to make the whole thing feasible," said Dr. Ronald S. Weinstein, chairman of the pathology department at Rush-Presbyterian-St. Luke's Medical Center in Chicago and founder and chairman of Corabi Telemetrics.

He said preliminary data from studies at Rush-Presbyterian show "no significant difference" in the accuracy of pathologists' diagnoses in breast cancer cases, whether they see the microscope slide directly or on a television screen.

Telepathology could mitigate doctor shortages in rural and inner city areas, expedite medical education and improve quality of care by giving community hospital patients access to immediate consultations from experts at major medical centers, Weinstein said. And it could reduce the risk to some patients by providing an immediate second opinion on a biopsy during surgery, as in the demonstration case, thereby eliminating the need for a second surgery.

"For rural medicine, this would be a boon," said Dr. William B. York, deputy commander for clinical services at Beaumont Medical Center in El Paso. Beaumont is a regional referral center for eight Air Force and Army hospitals in the Southwest, only two of which have a staff pathologist, as well as federal prisons, Indian reservations and the White Sands Missile Range in New Mexico.

Most small, rural hospitals cannot afford to keep a pathologist on staff, let alone the two required to ensure a second opinion in complicated cases, York said.

"It's really not economical, when they're seeing 20 samples a week, and we at Beaumont are seeing 80 a day," he said.

Telepathology has "tremendous potential" for the military, said Dr. Vernon Armbrustmacher, a pathologist and deputy director of the Armed Forces Institute of Pathology at Walter Reed Army Medical Center.

The institute, founded during the Civil War, has a staff of 140 physicians who annually review about 60,000 rare or difficult cases referred from hundreds of military and civilian hospitals around the world. Usually, when a doctor refers a case to the institute for an expert consultation, a sample of tissue is sent through the mail.

In complicated cases referred from distant hospitals or military bases, Armbrustmacher said, it can take two or three weeks to get a full pathology report back to the referring doctor.

"By that time, it's a question of whether the answer is timely enough," he said.

Local pathologists attending last week's demonstration said they were impressed with the clarity of the image and saw great potential for telepathology, despite uncertainty about its cost.

"It was very exciting," said Dr. Vivian Pinn-Wiggins, who chairs the pathology department at the Howard University Medical Center. "What was most impressive from the pathologist's point of view was that you can have a piece of tissue under a microscope in Texas and an image in Washington so clear that you see the microscopic detail on the monitor well enough to make an immediate diagnosis."

"I think there's a need for it," said Dr. Steven G. Silverberg, director of anatomic pathology at George Washington University Medical Center, adding that he was particularly impressed with the system's ability to let the pathologist "actually move the slide around by remote control."

Silverberg, former director of the Colorado regional cancer center in Denver, said telemedicine could be particularly useful in such areas, where a central hospital is responsible for complicated cases referred by a network of remote community hospitals.

Now that most of the key technical challenges have been overcome, Silverberg said, the major question is cost.

"This is big bucks," he said. "It's not going to cost nothing."

Detailed cost estimates aren't available yet, but Weinstein said he hopes the high initial costs will decline as the new market develops. Also, the same system that makes long-distance pathology consultation possible could be used to transmit X-rays, electrocardiograms and other medical images.

The cost of the specially designed microscope runs between $25,000 and $35,000, about double the cost of a comparable model without the video camera and computer hookup, said Beaumont's York. The communications equipment might run as much as $100,000, but York predicted that the first video-linked telepathology networks would be set up within three years.

"Commercially, we're talking about something in the $100,000 range" for the complete system, Weinstein said.