A 65-year-old woman with a previous diagnosis of bipolar affective disorder type II treated with lithium and olanzapine attended the emergency department for behavioural alterations of four days' duration. The patient has a long history of several admissions to psychiatric services. She lives alone and is independent in her activities of daily living, although she receives home help. The family reports initial symptoms of mood swings and extravagant dress, which later evolved into dysarthria, difficulty in walking and temporo-spatial disorientation. In the study performed in the Emergency Department, arterial hypertension (174/110 mmHg), hypokalaemia (3.09 mEq/l), fever (38.4ºC) and abdominal distension were observed, which was attributed to urinary retention, and 2 litres of urine were extracted after bladder catheterisation. Lithemia, ECG, CK and cranial CT scan were normal, as were plain abdominal and chest X-rays. Lumbar puncture was performed with normal cell count. The patient was admitted to the Internal Medicine Department with a diagnosis of confusional syndrome with no known aetiology. She was treated with antipyretics, broad-spectrum antibiotics and venous thrombosis prophylaxis with heparin. The study was extended with a new lumbar puncture, cranial MRI, thoracic and abdominal-pelvic CT scan, blood and urine cultures, which showed no abnormalities that could justify the symptoms. Within two days, the patient developed mutism, akinesia, rigidity, distal tremor and decreased blinking. Fever and arterial hypertension (AHT) persist, with hypernatraemia (150mEq/l). Electrolyte disturbances are interpreted in the context of diaphoresis and no fluid and food intake. After deterioration of the level of consciousness, admission to the Resuscitation Unit was assessed and a nasogastric tube was placed for the patient's nutrition. Psychiatric assessment was requested and electroconvulsive therapy (ECT) was prescribed urgently for the following day with the diagnostic impression of malignant catatonia. Two days after the first session, the patient was found to have coherent spontaneous speech and was feeding. Two more sessions of ECT were administered and lithium was reintroduced. Fever, diaphoresis, hypertension, electrolyte disturbances and disorientation have subsided. The culture for Herpes Simplex Virus was negative, ruling out encephalitis due to this cause. The patient was discharged in 30 days with a diagnosis of TAB type II and malignant catatonia. At discharge the patient was euthymic.