Measurement and monitoring of magnesium (Mg) are important to prevent the development of serious and potentially fatal complications in critically ill patients. Although ion-selective electrodes are available and earlier reports suggest that free ionized magnesium (iMg2+) is the most useful test to estimate Mg status, most clinical laboratories still only measure total Mg. To compare the relationship among iMg2+, total Mg, and albumin in serum, samples were collected from 48 consecutive patients admitted to an intensive care unit or a primary health center. The mean serum level of iMg2+ in 44 patients was 0.53 mmol/L, the total Mg was 0.96 mmol/L, and the albumin was 34.93 g/L. The correlation between iMg2+ and total Mg in serum was r=0.585; the correlation between iMg2+ and albumin in serum was r=378; and the correlation between total Mg and albumin in serum was r=0.340. The mean percent iMg2+ in relation to total Mg in serum was calculated to be 55% in the patient samples. The important level of biologically active iMg2+ was not reflected upon analysis of total Mg in 25% of consecutive patients. This report shows that the correlation of iMg2+ and total Mg is weak, not only in critically ill patients but also in patients in whom Mg status is inquired as a whole.