Simple liver cysts are common, rarely causing significant morbidity or mortality. Budd-Chiari syndrome (BCS) is caused by obstruction of hepatic venous outflow and is the leading cause of postsinusoidal liver failure. We present a rare case of BCS caused by a simple hepatic cyst. A 16 cm × 16 cm liver cyst was found on computed tomography of a 66-year-old woman presenting with abdominal pain. The cyst had become infected, thus enlarged, exerting mass effect with almost complete compression of the inferior vena cava. Shortly after admission, the patient developed acute liver failure, with deranged clotting and hepatic encephalopathy requiring full organ support on the intensive care unit. Cardiac output studies showed a low cardiac index of 1.4 l/min/m(2). An emergency laparotomy with fenestration of the cyst and drainage of 2l of purulent material led to a full recovery. Intraoperative cystic fluid aspirates later confirmed no evidence of Echinococcus. Histology confirmed a simple cyst. Liver biopsies showed severe, confluent, bridging necrosis, without background parenchymal liver disease. Acute BCS due to rapid compression of all major hepatic veins leading to fulminant hepatic failure is rare. Our case highlights a clinically significant complication of a simple liver cyst of which clinicians should be aware when managing these 'innocent' lesions.