Aim: Ruptured aortic aneurysm can be a cause of acute abdomen. Nowadays using modern diagnostic techniques diagnosis is very easy. We report a particular case of a misdiagnosed ruptured aortic aneurysm. Case Report: Man 65 y.o. admitted to hospital for abdominal pain and stipsis. After 1 week he developed acute abdomen with muscolar rigidity and mild acute anemia. Few days before contrast studies showed a sigmoid diverticular disease. At laparotomy a ruptured aortic aneurysm sealed from retroperitoneal fascia and extended to abdominal walls up to rectus abdominis muscles was found. Conclusions: In case of acute abdomen with muscolar rigidity and acute anemia the diagnosis of rectus abdominis muscles hematoma caused by ruptured aortic aneurysm must be considered.