This report describes a patient admitted to the intensive care unit (ICU) with extreme hypomagnesaemia (0.03 mmol/l). Profound weakness and rhythm disturbances were the most pronounced symptoms. This patient exhibited six risk factors for hypomagnesaemia: chronic dysphagia, acute diarrhoea and vomiting, high alcohol intake, use of a proton pump inhibitor and a diuretic. Despite this extreme hypomagnesaemia, he survived hospital admission. In order to determine the incidence of severe hypomagnesaemia (< 0.40 mmol/l) and its risk factors and manifestations, we searched our ICU patient database which included 21,296 patients. The incidence of severe hypomagnesaemia on ICU admission was 0.24%. Patients with severe hypomagnesaemia had a 44% incidence of sepsis and arrhythmia; 23% presented with a recent history of alcohol abuse, 36.5% had used a proton pomp inhibitor, half had acidosis and hyponatraemia, and 97% had low ionised calcium. Mortality lower than predicted. Although the literature reports that hypomagnesaemia occurs in up to 60% of ICU patients, severe hypomagnesaemia (< 0.40 mmol/l) was rare in the present population. Despite the seemingly crucial metabolic role of magnesium, severe hypomagnesaemia was not significantly associated with mortality in our unit. © 2014, Netherlands Society of Intensive Care. All rights reserved.