It is unclear whether insulin-dependent diabetes mellitus is a state of magnesium depletion. This is a relevant question, since magnesium deficiency has been implicated in the pathogenesis of diseases that develop to an increased extent into diabetes mellitus. Total plasma magnesium was not statistically different in 76 pediatric patients with insulin-dependent diabetes mellitus (0.77 [0.73-0.81] mmol/l; median and interquartile range), 59 healthy adults (0.80 [0.77-0.83] mmol/l) and 19 healthy children (0.80 [0.78-0.83] mmol/l). In contrast, plasma ionized magnesium, the most interesting form with respect to physiological and biological properties, was significantly lower in diabetic patients (0.50 [0.48-0.53] mmol/l) when compared with healthy adults (0.53 [0.50-0.56] mmol/l; p < 0.01) and healthy children (0.54 [0.51-0.56] mmol/l; p < 0.02). Our report confirms recent findings of reduced circulating ionized magnesium but normal circulating total magnesium in adults with non-insulin-dependent diabetes mellitus.