Men who order their burgers well-done double their cancer risk, a new study finds. Compounding this potential hazard is the fact that when meat is cooked at high temperatures over an open flame, cancer-causing chemicals are produced.

In the study of nearly 1,000 men, half of whom had recently been diagnosed with aggressive prostate cancer, those who consumed well-done grilled or barbecued meat twice weekly the previous year were over two times as likely to have prostate cancer as the men who ate none. Ground beef had the highest cancer risk, followed by processed meats and liver – but not so for poultry, bacon, low-fat hot dogs, sausages, or ground beef prepared medium or rarer, according to researchers at the University of California, San Francisco.

“This is another piece of evidence for the notion that red meat, particularly grilled meat, contains carcinogens that may relate to prostate cancer,” said Ronald D. Ennis, M.D., director of radiation oncology at St. Luke’s Roosevelt Hospital Center in New York, who was not involved in the study.

About one-fifth of all men in the U.S. will develop prostate cancer at some point in their lives. Existing risk factors include age, family history, smoking and body mass index.