 Guaiac faecal occult blood testing gFOBT consistently demonstrates reductions in deaths from colorectal cancer of around 16 and gFOBT screening is now routine in all four countries of the United Kingdom. However, gFOBT has significant limitations and is associated with a substantial interval cancer rate in the region of 50 , indicating a severe deficiency in sensitivity for cancer. Additionally, as the majority of colorectal cancers arise from pre-existing adenomas, it is important for colorectal screening programmes to detect adenomas in order to reduce the incidence of the disease as well as the associated mortality. Although gFOBT does detect some adenomas, most randomised trials have not demonstrated a reduction in colorectal cancer incidence. Also, FOBT screening tends to under-detect cancers in women and it is relatively insensitive for rectal cancer when compared with colon cancer. 