Lower gastrointestinal hemorrhage is a common clinical problem for which multiple diagnostic tests and therapeutic interventions have been developed but no optimal approach has been established. We reviewed 107 consecutive patients admitted to the Massachusetts General Hospital for management of acute lower gastrointestinal hemorrhage to determine the effectiveness of diagnostic and management technologies, with particular attention to urgent colonoscopy. Colonoscopy yielded a diagnosis in 90% of patients, provided the opportunity for successful therapy in 9 of 13 patients (69%), and shortened hospital stay. Angiography performed after a scan positive for bleeding was often diagnostic, and angiography provided the means for successful therapy in 5 of 10 patients (50%). Barium enema and sigmoidoscopy had lower clinical yields. Although roles exist for other technologies, colonoscopy is the most convenient and effective first test in the evaluation of patients with significant lower gastrointestinal hemorrhage. Diagnostic yield, therapeutic opportunity, and cost effectiveness are maximized in early studies.