Infertile men are nearly three times more likely to develop testicular cancer than those who are fertile, U.S. researchers reported on Monday.

The finding suggests a common source for both problems, perhaps errors in the way the body tries to repair damage to genetic material or environmental factors, Dr. Thomas Walsh and colleagues at the University of California, San Francisco reported.

While the infertility-cancer link has been found in Europe, the new study — which looked at more than 22,000 California men — is the largest U.S. investigation so far, the research team reported in the Archives of Internal Medicine.

The men in the study had been evaluated for treatment at infertility treatment centers. Those found to be infertile "were 2.8 times more likely to develop testicular cancer relative to the general population," the researchers wrote.

Walsh, now at the University of Washington School of Medicine in Seattle, and his fellow researchers said they did not believe that infertility treatment was at the root of the problem. Men do not get surgery or drugs for infertility.

"A more plausible explanation is that a common exposure underlies infertility and testicular cancer," they wrote. "Prior work ... suggests that certain severe forms of male infertility are associated with faulty DNA repair," which is also associated with the development of tumors."

In addition, they said, other research has suggested an interplay involving both genetics and environmental factors.

During the last 30 to 50 years, the researchers said, there has been a notable and continued increase in the incidence of testicular germ cell cancers among men, especially in Scandinavian countries.

"During the same period there is evidence of a decline in semen quality and fertility in industrialized nations," they wrote. "It is unclear whether these two trends are independent or related to one another."