A decrease in serum Mg(+2) is associated with airway hyper-reactivity and impaired pulmonary function. The purpose of this study was to determine if decreased serum Mg(+2) levels in patients with chronic obstructive pulmonary disease (COPD) are associated with acute exacerbations. In a retrospective study, the charted serum Mg(+2) levels in 100 COPD patients were examined. These included 50 patients who presented with an acute exacerbation of COPD and 50 stable patients. Chart review was sequential within both groups. Serum Mg(2+) levels in the stable COPD patients averaged 0.91+/- 0.10 mmol/L (mean+/- SD) with a 95% CI of 0.88 - 0.94 mmol/L. Patients undergoing an exacerbation had significantly lower serum Mg(+2) levels (0.77+/- 0.10 mmol/L; CI, 0.74 - 0.79; p<0.0001). Logistic regression of the dichotomous outcomes as a function of serum Mg(+2) concentration demonstrated a highly significant association (chi(2)=41.26; p<10(-5)). These data were subjected to receiver-operator characteristic (ROC) analysis for decision levels (DL) and the area under the ROC curve was determined to be 0.85+/- 0.04 (CI, 0.78 - 0.93). The optimum DL was determined to lie between 0.80 mmol/L (OR=14.33; sensitivity 70%; specificity 86%) and 0.84 mmol/L (OR=11.16; sensitivity 84%; specificity 68%). These data suggest that at the lower range of the reference interval, serum Mg(+2) levels are associated with an increased risk of exacerbation of symptoms in COPD patients. Furthermore, they suggest a DL that is useful for predicting clinical outcomes in these patients and serving as a target value for therapy.