Objective: To report a case of unusual severe adverse reaction to an angiotensin-converting enzyme inhibitor (ACEI). Design and method: A 33-year-old male with a history of hypertension for 10 years, asthma, and atopy was admitted to a local emergency department due to diffuse abdominal pain with vomiting and diarrhea for about 3 days. A few days earlier, the patient had blood pressure (BP) lowering treatment modified due to suboptimal BP control, with introduction of perindopril. The first ACEI dose was followed by lower abdominal pain, urge to have bowel movement, and difficulty passing stool. 24 hours later, the next dose was followed by severe, gradually increasing, diffuse abdominal pain, nausea, vomiting, and diar-rhea. With signs of peritonitis present on admission, a surgeon suspected an acute abdomen and or-dered abdominal computed tomography (CT) due to unclear etiology. CT showed diffuse infiltrates within the small intestine and its mesentery, with free abdominal fluid but no perforation. The patient was not operated and seen by an internist who suspected gastrointestinal angioneurotic edema related to ACEI. On admission to the medical unit, the patient was moderately unwell, conscious, with BP 185/110 mmHg, HR 128 bpm, SaO2 99% on room air, and no fever. Abdomen examination showed reduced bowel sounds, abdominal guarding, tenderness on palpation, especially in the mid-abdomen, and rebound tenderness. Laboratory tests showed elevated WBC (16,800/mm3) and CRP (19 mg/L). Results: Perindopril was withdrawn, the patient received antihistamines, analgesics, and intravenous fluids, with improvement of the patient's condition and reduction of pain and inflammation markers. Follow-up abdominal CT after 5 days showed resolution of small intestinal edematous lesions. The patient was discharged home in good condition, without abdominal pain, and with good BP control. The final diag-nosis was gastrointestinal angioneurotic edema associated with ACEI treatment. Conclusions: One condition may mimic another and thus unclear clinical presentations require cautious diagnostic and therapeutic decisions. In the presented case, angioneurotic edema following introduction of perin-dopril was unusually located in the gastrointestinal tract and its manifestations imitated an acute abdo-men.