Acute lower gastrointestinal (LGI) bleeding is typically caused by vascular malformations, diverticuli and neoplasia. Although endoscopic evaluation of the colon is relatively standard in stable patients with LGI bleeding, those with significant ongoing hemorrhage are often more difficult to evaluate endoscopically. Other investigative techniques such as nuclear scintigraphy, angiography and surgical exploration have been commonly used in unstable patients with LGI bleeding when the exact site is unknown. These investigative techniques have had variable measures of success. This two-part review evaluates the literature in an attempt to review the optimal investigative approach in patients with LGI hemorrhage, in particular patients who have had significant and ongoing bleeding. Part 1 of this article concentrates on the etiology of LGI hemorrhage, followed in a subsequent article by diagnostic and management strategies. Following the review, a consensus update will be included with guidelines for clinical use.