Men who carry a faulty gene have a one in 15 chance of developing breast cancer by the time they are 70, researchers said today.
A faulty copy of the gene - called BRCA2 - causes hundreds of UK cases of breast cancer in women every year, and can be passed on to both sexes through the generations.
Breast cancer is rare in men, with about 300 cases in Britain each year compared with more than 45,000 among women.
Now, in the largest study of its kind, researchers have worked out the lifetime risk of breast cancer for men who carry the faulty gene.
Previous studies have linked the gene to prostate and pancreatic cancer in men.
Women with a faulty copy of the gene have a much higher risk of ovarian cancer, and increased risk of other cancers including cervical, uterine and bowel cancer.
The research, published in the Journal of Medical Genetics, involved data from 321 families with the faulty BRCA2 gene living in and around Manchester and Birmingham.
Among the families, 16 men who were first degree relatives of known BRCA2 carriers had developed breast cancer between the ages of 29 and 79. Another eight cases occurred in second degree relatives.
Professor Gareth Evans, from St Mary's Hospital in Manchester, who led the study, said the vast majority of the men who developed breast cancer were BRCA2 fault carriers themselves.
Analysis showed the risk of breast cancer among men with a faulty BRCA2 gene was 7.1% by the age of 70 and 8.4% by the age of 80.
Prof Evans said: "There is a one in 1,000 chance of developing breast cancer as a man in the general population.
"Men don't have to have a faulty copy of BRCA2 to get breast cancer, but the highest risk for man is if they have a faulty copy of the gene.
"The risk is 90 times higher than the risk for men in the general population."
Writing in the journal, his team said: "These risks are sufficient to increase awareness of breast cancer among men in BRCA2 families and to stress the importance of early presentation with breast symptoms."
Arlene Wilkie, director of research and policy for the Breast Cancer Campaign, said breast cancer is rare in men.
"Symptoms include a lump or thickening in the breast area and men should watch out for changes in the skin covering the breast such as a rash, ulceration or skin dimpling. Nipple discharge or inversion can sometimes occur too.
"Breast tissue extends into the armpit so anything unusual in this area should also be checked by a GP.
"Most lumps turn out to be harmless, but breast cancer is very treatable when detected early so if you have any of these symptoms, visit the GP without delay."