Men who are circumcised early - before their first sexual intercourse - have a lower risk of prostate cancer, according to a new study published in Cancer, the American Cancer Society's journal.
Scientists think circumcision may reduce infection and inflammation, which are linked to cancer.
Lead researcher Jonathan Wright, of the University of Washington in Seattle, began to explore a possible link between circumcision and prostate cancer after studies linked circumcision with a drop in the spread of AIDS.
"My interest came from seeing the randomized trials from Africa, showing a reduction in not just HIV but also several other sexually transmitted diseases with those that are circumcised, recognizing that several sexually transmitted diseases have been associated with a higher risk of prostate cancer."
So maybe, Wright thought, circumcision might help prevent prostate cancer.
To find out, he compared two groups of men - about 1,700 prostate cancer patients, and a similar number of randomly selected non-patients. Researchers asked them all if and when they were circumcised, and then analyzed the results.
"Those men who were circumcised before their first sexual encounter had about a 15 percent reduction in the risk of developing prostate cancer," Wright said. "This was true in both Caucasians and African-Americans; it was true for both more-aggressive and less-aggressive cancer, so it appeared quite consistent across the board."
Wright points out that his study shows an association - that circumcised men are less likely to get prostate cancer - but not necessarily that being circumcised is the cause of the lower cancer rate.
So what might explain the fact that the circumcised men in this study had fewer prostate cancers?
Wright suggests two possibilities. One is that the presence of the foreskin may promote inflammation. Or it may be that uncircumcised men are more likely to pick up a sexually transmitted infection that could increase the risk of prostate cancer.
His paper cites a study that found that about one out of six cancers worldwide are caused by infections.
"We do know both pathways are important in developing cancers. Which one in this case? We can't say."