Aim To estimate the effectiveness of colorectal cancer screening with faecal occult blood testing (FOBT), flexible sigmoidoscopy (FS), and combinations of FOBT and FS in preventing colorectal cancer (CRC) deaths. Method A systematic review was conducted examining randomised controlled trials (RCTs) published between 1997 and 2004 inclusive. A systematic search of Medline, Embase, Current Contents, and the Cochrane Library was undertaken. Studies that evaluated screening with FOBT, FS or combinations of FOBT and FS, were appraised. A meta-analysis of population-based trials of FOBT was conducted. Results Four RCTs were identified that examined FOBT screening. The three trials that investigated guaiac-based FOBT found CRC mortality was reduced in the screening group. In the two population-based trials, the pooled relative risk was 0.86 (95%CI 0.79–0.93). A fourth RCT was identified, with shorter term follow-up, which considered FOBT screening combined with FS compared with FOBT alone. No significant reduction in CRC mortality was reported in this trial. Conclusion There is high-quality evidence showing that guaiac-based FOBT screening reduces mortality from CRC. No such evidence exists for screening with FS either alone, or in combination with FOBT, but this should be re-evaluated once data become available from four large ongoing trials.