Ca and Mg are biologically important minerals that are involved in maintaining the stability of membranes, muscle contractions, and enzyme activity in nearly all cells. Derangements of Ca and Mg homeostasis can precipitate serious and life-threatening problems in the critically ill patients. We examined serum ionized Ca and Mg levels in pediatric patients consecutively admitted to a pediatric ICU. Abnormal Mg and ionized Ca levels on admission were found in 43.3% and 17% of the patients, respectively. Hypocalcemia and hyper-magnesemia were both associated with poor outcome as measured by either survival or length of ICU stay. In addition, ionized Ca levels could not be predicted from total Ca measurement either alone or in combination with serum albumin and pH.