NEW YORK -- A computer is as good as a second pair of eyes for helping a radiologist spot breast cancer on a mammogram, one of the largest and most rigorous tests of computer-aided detection found.

Like spell-checkers looking for mistakes, the computers flag suspicious areas on X-rays for a closer look by a radiologist. Mammograms are used to screen women for early signs of breast cancer, but the tests aren't perfect. In the U.S., the X-rays are read by a single radiologist, and cancers are sometimes missed.

Computer-aided detection, or CAD, was developed to help radiologists pick up more cancers. Approved a decade ago, these computer programs are now used for about a third of the nation's mammograms. But the value and accuracy of the technology has continued to be debated.

Now, British researchers are reporting results from a randomized study of 31,000 women. Mammograms in Britain are routinely checked by two radiologists or technicians, which is thought to be better than a single review. Researchers wanted to know if a single expert aided by a computer could do as well as two pairs of eyes.

They found that computer-aided detection spotted nearly the same number of cancers, 198 out of 227, compared to 199 for the two readers.

In places like the United States, "Where single reading is standard practice, computer-aided detection has the potential to improve cancer-detection rates to the level achieved by double reading," the researchers said. Their findings were published online last week by the New England Journal of Medicine.

The study was done at three centers in England that do a large number of routine mammograms. Most of the women in the study were assigned to have their mammograms reviewed twice -- once by a pair of experts and a second time by a single reviewer aided by a computer.

"What we demonstrated was that one reader using CAD could pick up as many cancers as the two readers could," said radiologist Fiona J. Gilbert of the University of Aberdeen, lead author of the study.

She said computer-aided detection could be used to expand screening by Britain's national health service, which now offers the test every three years to women 50 to 70. The cost-effectiveness will have to be determined first, she said.

The new findings are encouraging, said Dr. Carol H. Lee, a radiologist at Memorial Sloan-Kettering Cancer Center in New York.

"In the United States, it's just not practical in most practices to do double readings by physicians," said Lee, who is head of the American College of Radiology's Breast Imaging Commission. "These results are reassuring to me that single reading with CAD can achieve that same sensitivity."

The U.S. government recommends mammograms every one or two years starting at age 40. Experts said there aren't enough radiologists to give mammograms two readings, and insurers don't pay for a second look. Medicare does pay an additional $15 for computer-aided detection.

That extra money helped spur the adoption of the computer checks, said Dr. Ferris M. Hall, a radiologist at Boston's Beth Israel Deaconess Medical Center, where computers are used. He expects more places to use them as they switch to digital images from film X-rays, which eliminates a step in the process.

The research was funded by the British government and Cancer Research UK, a charity. Two of the researchers have received fees from the maker of a computer system and served as unpaid consultants to another.

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