The onset of malignant hyperthermia in a patient during a prolonged anaesthetic for tumour resection is described. The onset was delayed with a gradual rise in heart rate and PetCO2 before becoming fulminant; muscle rigidity was not a feature. Other aspects of the patient’s condition confused the presentation, delayed the diagnosis and may have been involved in precipitating the event. However, it responded rapidly to treatment and surgery was continued. A possible recrudescence occurred 18 h later. Malignant hyperthermia should be considered early in cases of unexplained tachycardia or rising PetCO2.