Background: Nearly 10% of emergency consultations are due to acute abdominal pain. In people over 65 years old, it can have atypical presentations, that retard the correct diagnosis and worsens prognosis. Aim: To study the causes, evolution and prognosis of acute abdomen in the elderly. Material and methods: Prospective study of 45 patients aged more than 65 years old (mean age ± SD, 75.7±7.7, 51% men) and 221 patients of less than 65 years old (mean age ± SD, 36.7±14.0, 48% men), consulting for acute abdomen in the emergency room. Results: Sixty six percent of elderly patients had concomitant diseases, that were multiple in 63%. In this age group, the causes accounting for 71% of acute abdominal pain were bilio-pancreatic diseases (31.1%), intestinal adhesive obstruction (17.7%), complicated abdominal wall hernia (13.7%), and complications of peptic ulcer disease (8.9%). Sixty four percent required surgical treatment and, in almost 50% the surgical risk was classified in ASA III or IV, according to the American Society of Anesthesiology. Thirty one percent had postoperative complications. Compared with their younger counterparts, elderly patients required significantly (p<0.05) more admissions to intensive care units (2.7 and 24.2% respectively), more connections to mechanical ventilation (1.4 and 8.9% respectively) and longer hospital stays (5.4±7.4 and 12.4±10.9 days, respectively). In this series overall mortality was 6.7%, being 0.6% for young patients and 11.1% for the surgical group over 65 years old. Conclusions: Acute abdomen in the elderly has a high rate of complications and mortality. According to the causes of acute abdomen in this group, evaluation in the emergency setting with an ultrasonography may be very useful. In the elderly, elective correction of potential causes of acute abdomen should be done.