This is from an article by Gina Kolata of the New York Times:

The P.S.A. blood test, the popular screening test for prostate cancer, saves few if any lives and exposes large numbers of men to risky and unnecessary treatment…..

The European….. studies found that screening was associated with a 20 percent relative reduction in the prostate cancer death rate. [link; gated, but with abstract.] But the number of lives saved was small: 7 fewer prostate cancer deaths for every 10,000 men screened and followed for nine years…..

The American study….. found no reduction in deaths from prostate cancer after most of the men had been followed for 10 years. [link; gated, but with abstract.]

The reason screening saved so few lives, cancer experts say, is that prostate cancers often grow very slowly, if at all, and most never endanger a man if left alone…..

Mammography has about the same effect as the P.S.A. test.