Background: Circulating magnesium exists in the ionized state and in the undissociated form, either bound to albumin, or complexed to various anions. Until recently, only the measurement of total plasma magnesium has been possible. Now circulating ionized magnesium can be assessed as well. Methods: Total and ionized plasma magnesium were determined in 43 patients on maintenance hemodialysis (dialysate composition: calcium 1.75 mmol/l, magnesium 0.75 mmol/l) before a dialysis session and in a group of 23 healthy subjects. Results: The total (1.16 [1.03–1.31] versus 0.81 [0.78–0.89] mmol/l; median and interquartile range) and the ionized (0.71 [0.66–0.78] versus 0.54 [0.53–0.59] mmol/l) plasma magnesium levels were significantly higher (p < 0.01) and the ionized plasma magnesium fraction lower (0.61 [0.58–0.65] versus 0.67 [0.64–0.70]; p < 0.02) in patients than in controls. Conclusion: The determination of circulating ionized magnesium using selective electrodes is an attractive method to evaluate extracellular magnesium in kidney disease.