But the study?s authors warn that young men and teenage boys shouldn?t alter their physical activity or change jobs as a result of these findings. "This is just one study," said co-author Nancy Kreiger, PhD, senior epidemiologist with Cancer Care Ontario in Toronto. "One result isn?t enough."

Dr. Kreiger and lead author Anil Srivastava, with the department of health sciences at the University of Toronto, published their findings in a recent issue of the American Journal of Epidemiology (Vol. 151, No. 1).

The two researchers compared 212 men with testicular cancer with 251 men who did not have the disease. The men filled out questionnaires on the frequency of moderate or strenuous exercise during specific periods of their lives. The researchers were surprised to discover a relationship between moderate and strenuous physical activity and testicular cancer.

"Our study found evidence that a relatively high frequency of participation in moderate and strenuous recreational activity in one?s mid-teens may have an adverse effect on risk of testicular cancer," the authors wrote. "Moderate or strenuous occupational demands in one?s 20s also increased risk of disease."

The study showed men who had moderate or strenuous job demands in their 20s had a 70 percent to 85 percent increased risk of testicular cancer, compared to those in the cancer-free group who did not report having similar jobs in their 20s.

The authors said the findings contradicted their original belief that physical activity would have a beneficial effect on testicular cancer risk. But they acknowledge more studies need to be done before changes in physical activity levels can be recommended. "Important insights might be gained from further investigation of the relation between hormone levels and physical activity, particularly with respect to frequency of exercise," the researchers wrote.

Michael Thun, MD, vice president of epidemiology and surveillance research for the American Cancer Society (ACS), said it is important to realize this is only one study and should be interpreted as raising concern rather than providing proof. Even if it is true, testicular cancer is rare compared to other cancers for which physical activity is beneficial, he added.

However, this study will help researchers in their quest to understand testicular cancer, Dr. Thun said. "The increase in incidences of testicular cancer between 1973 and the late 1980s in the US, Europe, and Australia has been a fundamental puzzle," he said. "Any clues are important."

He agreed with the study?s authors and advised people not to place too much weight on the study?s findings just yet. "It?s too early," Dr. Thun said.

It's also important for young men to realize that this study deals with only one type of cancer, and a very rare one at that. An average man's lifetime risk of dying from testicular cancer is about 0.02 percent (1 in 5,000).

Although the study suggests, but doesn't prove, that strenuous physical activity may increase this risk, other studies have found that exercise lowers a man's risk for several other cancers.

These studies suggest that physical activity can lower risk of developing and dying of cancers of the colon, rectum, and perhaps other organs. Physical activity may lower risk of these cancers by directly affecting metabolism, and/or by helping men avoid obesity, which may lead to metabolic changes that increase cancer risk. Unlike testicular cancer, which is quite rare, colorectal cancers are more common. Lifetime risks for dying of colorectal cancer is about 2.45 percent (1 in 41). And, of course, physical activity has an even greater impact on lowering the risk of developing or dying of heart disease, stroke, and diabetes. For this reason, the ACS recommends that both men and women be physically active for at least 30 minutes on most days.