“It’s as if we have been there before, and it’s not a surprise,” says Gary Steinberg, a Stanford Medicine neurosurgeon who helped create the school’s two-year-old Neurosurgical Simulation and Virtual Reality Center.

Stanford Medicine is just one of a growing number of hospitals and medical schools embracing virtual technology. The goal is to provide better and faster training for resident doctors and surgeons, whose skill can mean the difference between life and death for their patients.

Virtual reality’s adoption in medicine comes after major improvements to the technology over the past few years. Early headsets and software provided jerky imagery that nauseated some users, ruling it out for medical training.

Since then, the technology has improved so much that medical students can learn anatomy by walking around a lifelike digital hologram of a lung and transport themselves inside a heart to see the valves and pumping blood.

Mark Griswold, radiology professor at Case Western Reserve University’s Case Center for Imaging Research in Cleveland, says virtual technology is a big help in teaching the next generation of doctors. Students who used Microsoft HoloLens VR headsets to learn part of the human anatomy—an arm, for exam- ple—acquired that knowledge in nearly half the time compared with students who studied the same area solely on cadavers.

Speedier learning is especially important in countries like China and India, which, combined, will need 6 million new physicians by 2020. Meanwhile, the U.S. needs an additional 20,000 surgeons to treat its aging population. The question is how to train all of them. Virtual reality may be the answer, although it comes with a hefty price tag.

Pharmaceutical giant Johnson & Johnson has spent millions of dollars on an initiative it began in 2017 to establish 24 surgical training centers, a few of which have virtual reality, worldwide. Though off-the-shelf virtual reality hardware is becoming cheaper, it’s still expensive to integrate with the necessary software that translates conventional medical images into 3D.

Courtesy of Johnson & Johnson

It costs Johnson & Johnson nearly $10,000 per VR unit on average. Stanford’s Neurosurgical Simulation Center, funded by its parent hospital and outside donors, cost $750,000.

Virtual technology comes in two flavors: a fully immersive experience, in which users see only a computer-generated environment; and mixed reality, in which 3D images are projected onto the physical world.

Physicians already use virtual technology for a variety of medical procedures, including cancer treatment, by creating interactive maps of tumors; and physical therapy, by having patients play games that encourage movement. But in surgery, it has perhaps the greatest potential.

Traditionally, medical students are judged by how long it takes them to perform a procedure. Instead, with VR, medical students can be graded on whether they make a mistake.

“It gives us a way to judge whether the medical student has learned what they are supposed to learn,” says Richard Satava, professor emeritus of surgery at the University of Washington in Seattle.

At Stanford, the Neurosurgical Simulation Center resembles a miniature movie theater, including four cinema-quality seats (complete with cup holders) for students and surgeons to sit in while using VR. Spectators can watch on large TV screens mounted on the wall.

In addition to doctors and students, 400 neurosurgery patients have viewed their surgeries in virtual reality before their procedures. “They can immerse themselves in their brain,” Stanford’s Steinberg says. “It puts them at ease and shows them exactly what we’re going to do.”

A version of this article appears in the January 2019 issue of Fortune with the headline “VR Gets Real In The OR.”

Correction: An earlier version of this article mistakenly described the number of Johnson & Johnson-sponsored surgical training centers that include virtual reality. Only a few of the 24 centers feature the technology.