Assessment of magnesium (Mg) status by serum and 24-hr urine estimations has been used to study a group of 17 patients with severe Crohn's disease, 10 of whom have required intravenous (IV) nutrition. Mg depletion was present in 15 (88%) and severe enough to cause symptoms in two. Urine levels were low in most patients and would appear to be a more sensitive indication of Mg depletion than serum levels alone. Serum Mg levels were significantly lower in the Crohn's group than in a group of hospital controls; 5-10 mmol of IV Mg were required daily to prevent Mg depletion during IV nutrition and some patients required higher intakes. Three patients with particularly severe malabsorption required oral Mg supplements in the long-term. The rationale for using our method of assessing Mg status, and the importance of recognizing and treating chronic Mg deficiency are presented.