Colorectal cancer: have we identified an effective screening strategy? Three currently used screening methods are aimed at detecting colorectal cancer when it is asymptomatic and curable, and at detecting polyps so that they can be removed before they can progress to cancer. Digital rectal examinations are relatively cheap and easy but can detect only a small fraction of large-bowel cancers. Sigmoidoscopy is more sensitive, but its low acceptability to patients has been only partially mitigated by the introduction of the 35-cm flexible instrument. Fecal occult blood testing has limited sensitivity because blood from cancers and polyps is neither continuously shed nor uniformly distributed in feces; specificity and positive predictive value are also low because of other sources of blood in the stool. Prudent judgment suggests that all of these screening tests may prevent death from colorectal cancer in some patients. However, none has been proven effective in general use by well-controlled studies. Case-control studies can provide timely and valuable new evidence in this regard; the authors' investigations in progress are described. The current lack of strong evidence in support of these screening tests should not be interpreted as evidence against their use. 