Fecal occult blood testing is used worldwide to screen for early detection of colorectal neoplasia. Because processing Hemoccult II cards is not difficult, it is very easy for inexperienced personnel to perform, and opens a way to overdiagnose positive tests. This could be followed by unnecessary, expensive diagnostic procedures and a significant decline in the test specificity. In a prospective study of two similar populations (two kibbutzim of the Upper Galilee in northern Israel), I compared the impact of processing the Hemoccult II cards on their positive predictive value for colonic cancer and adenomatous polyps. The inexperienced processor increased overall screening positivity fourfold (25 versus 6%, p less than 0.01). This was accompanied by a sharp decrease in the predictive value for adenomas greater than 1 cm (6 versus 25%, p less than 0.05) with no change in the final screening yield: 1.58 versus 1.53% of the participants had adenomas in kibbutz A and kibbutz B, respectively. I recommend that the Hemoccult II cards be processed by experienced personnel only.