This study was designed to assess the effectiveness of identifying serum magnesium abnormalities by comparing physician-initiated requests for this analyte with routine magnesium determinations. Because magnesium abnormalities frequently accompany other electrolyte abnormalities, we measured magnesium in 1033 serum specimens submitted for electrolyte analyses. Physician-initiated requests for magnesium measurements were received for 81 (7.4%) of these specimens. Serum magnesium abnormalities were identified in 546 of the 1033 specimens (hypomagnesemia [less than 0.74 mmol/L], 487; hypermagnesemia [greater than 0.99 mmol/L], 59). Only 10% of the hypomagnesemic patients (48/487) and 13% of the hypermagnesemic patients (7/59) were identified by physician-initiated requests for this analyte. Fifty-three patients were both hypomagnesemic/hypokalemic and 30 patients were both hypomagnesemic/hyponatremic, but only 8 (15%) and 3 (10%), respectively, had physician-initiated requests for magnesium. Because magnesium abnormalities in significant numbers of patients are not being detected, we recommend routine measurement of this analyte when analyses of electrolytes are required for the care of patients.