Detection of gastrointestinal bleeding by means of radiotracers is a non-invasive method, and should be considered when endoscopy and barium radiography have failed to identify the focus. The two most prominent tracers are 99mTc sulphur colloid and 99mTc red blood cells, and both detect lower gastrointestinal bleeding with at least the same sensitivity as angiography. A case of gastrointestinal bleeding in a 68-year old woman is presented. The bleeding source in the colon was precisely identified with the 99mTc RBC method, and was later shown to be a metastasis from her endometrial cancer.