We present a case of a 65-year-old woman with fever, behaviour disorder and mutism. The differential diagnosis of fever and neuropsychiatric disorders requires a systematic study to get an early understanding of the aetiology, allowing us to provide the correct treatment. In our case, the early diagnosis of malignant catatonia enabled us to give our patient electroconvulsive therapy, achieving a spectacular improvement. In this article, we suggest an algorithm that could be useful in daily medical practice.