Acute lower gastrointestinal bleeding (LGIB) is a common disorder which involves many problems in diagnosis and treatment. Concerning colonoscopy in an urgent context, guidelines are very specific in stating that unprepared colonoscopy has no place in the evaluation of patients with suspected LGIB. The purpose of this study was to evaluate the diagnosis and therapeutic effectiveness of urgent colonoscopy, without prior bowel preparation, in patients admitted at the hospital with acute LGIB. Were included 132 patients consecutively admitted at the Emergency Department for LGIB and who had undergone colonoscopy without prior bowel preparation. Despite the lack of preparation, cecal observation was possible in 10 patients (7.6%). More than two thirds of the diagnoses (69.1%) were established with the performance of just a left colonoscopy. In 63.6% of patients, a definitive diagnosis was established using this initial approach. In 35.6% of the individuals, biopsies were performed, which allowed the establishment of a histological diagnosis. Additionally, in 14 patients (16.7%) an active hemorrhagic focus was identified, and endoscopic hemostasis was performed. None of these 14 patients required additional procedures during the hospital stay. Only in 21 patients the initial colonoscopy was inconclusive and, consequently, it was necessary to repeat it after anterograde preparation. Even in these cases, no diagnosis was reached in 33.3%. This study revealed that unprepared colonoscopy in an urgent context could a useful procedure which allows an adequate guidance in more than half of the patients.