Background. Almost 99% of the body magnesium is inside cells. The concentration of intracellular ionized magnesium (iMg) is physiologically relevant. iMg in erythrocytes is a new parameter that can help to establish reliable information on the functional magnesium status. Methods. iMg concentration in erythrocytes and serum was measured by ion-selective electrode, in clinical analyzer Microlyte (KONE). Total magnesium (tMg) concentration was measured by atomic absorption spectrometry (AAS). Albumin and total protein concentration were measured colorimetrically. Results. In critically ill postoperative patients, the mean of albumin, protein and hematocrit concentration was significantly lower compared to healthy individuals. Hypomagnesemia was found in 15.9% patients as tMg s, at 22.2% as iMg s and 36.5% as iMg e. Significant correlations are between iMg s and tMg s or iMg e and iMg s/tMg s. In dialyzed patients, the mean of hematocrit was significantly lower, iMg e was significantly higher compared with healthy individuals. Significant negative correlations are between iMg s and tMg e or iMg e/tMg e and tMg e. Conclusions. iMg e is the best magnesium parameter to observe hypo- or hypermagnesemia for both groups of patients. The function of magnesium is mainly intracellular and intracellular magnesium concentrations can be the method to evaluate the magnesium status.