BOSTON, MASSACHUSETTS. Researchers at the Harvard Medical School report that they have discovered a new diagnostic test which will predict men's risk of prostate cancer years before the cancer actually develops. Their discovery is part of the ongoing Physicians' Health Study which was started in 1982 and involves almost 15,000 physicians. By March 1992 520 cases of prostate cancer had occurred among the physicians. In 152 of these cases enough blood plasma had been collected in 1982 to perform analyses for the content of IGF-I (insulin-like growth factor-I). These analyses were done in 1997 and the results compared to analyses done on 152 plasma samples from 1982 obtained from physicians without prostate cancer. The researchers found that survey participants who later developed prostate cancer tended to have higher levels of IGF-I in their blood than did the controls. Men with IGF-I levels in the upper quartile (25 per cent) were found to have a 4.3 times greater risk of developing prostate cancer than did men with levels in the lower quartile of results. On average, there was a seven-year lag between the appearance of a high IGF-I level in the blood samples and the actual diagnosis of prostate cancer. The importance of high IGF-I levels was found to be particularly significant for men over 60 years of age. In this age group men with IGF-I levels in the highest quartile were almost eight times more likely to later develop prostate cancer than were men with levels in the lowest quartile. It is interesting to note that many of the men with elevated IGF-I levels who later developed prostate cancer had normal PSA levels (less than or equal to 4 ng/ml) when the blood samples were collected. The researchers conclude that IGF-I levels may serve as an early warning signal for prostate cancer in much the same way as high cholesterol levels serve as an early warning for atherosclerosis and heart disease. They also raise concern that administration of growth hormone and specifically IGF-I to delay the effects of aging in older men may increase the risk of prostate cancer. Other researchers (data still unpublished) have found an equally strong link between IGF-I levels and the risk of breast cancer and are currently investigating an association with colon cancer.