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Men with vasectomies may be at an increased risk for the most lethal form of prostate cancer, researchers have found. But aggressive cancer nonetheless remains rare in these patients.

Earlier studies had hinted at a connection between vasectomies and prostate cancer. Many experts have dismissed the idea of a link: Men who have vasectomies may receive more medical attention, they said, and therefore may be more likely to receive a diagnosis. The new study, published this month in The Journal of Clinical Oncology, sought to account for that possibility and for other variables.

Researchers at Harvard reviewed data on 49,405 men ages 40 to 75, of whom 12,321 had had vasectomies. They found 6,023 cases of prostate cancer among those men from 1986 to 2010.

The researchers found no association between a vasectomy and low-grade cancers. But men who had had a vasectomy were about 20 percent more likely to develop lethal prostate cancer, compared with those who had not. The incidence was 19 in 1,000 cases, compared with 16 in 1,000, over the 24-year period.

The reason for the increase is unclear, but some experts have speculated that immunological changes, abnormal cell growth or hormonal imbalances following a vasectomy may also affect prostate cancer risk.

Dr. James M. McKiernan, interim chairman of the department of urology at Columbia, said the lack of a clear causal mechanism was a drawback of the new research.

“If someone asked for a vasectomy, I would have to tell them that there is this new data in this regard, but it’s not enough for me to change the standard of care,” he said. “I would not say that you should avoid vasectomy.”

The lead author, Lorelei A. Mucci, an associate professor of epidemiology at the Harvard School of Public Health, emphasized that a vasectomy does not increase the risk for prostate cancer over all. “We’re really seeing the association only for advanced state and lethal cancers,” she said.

She agreed with Dr. McKiernan that the new data are not a reason to avoid a vasectomy. “Having a vasectomy is a highly personal decision that men should make with their families and discuss with their physicians,” she said. “This is one piece of evidence that should be considered.”