Acute appendicitis is the most common urgent surgical condition observed in patients over 65 years of age after biliary tract diseases and gastrointestinal obstructions. Ileo-caecal position of the appendix determines the possible clinical condition. The findings forperitonitis in retrocaecal appendicitis are indefinite, and diagnosis would be more difficult if advanced age and psychiatric disorders are also present in addition to these indefinite findings. A 67-year-old female patient with schizophrenia who had percutaneous drainage due to retroperitonal abscess was referred to our clinic. Diffuse retroperitoneal abscess and perforrated retrocaecalappendicitis were detected during laparotomy. The patient lost her life due to multiple organ failure after three weeks. As in the present case, an interrogator approach and an early diagnosis and treatment in old patients with psychiatric disorders would reduce the mortality.