Hypocalcemia is an important metabolic problem in critical care medicine. To determine the frequency of this problem and the patient subsets at risk, a retrospective study of a large series of ICU patients was performed. During the study period, 259 patients were admitted to the ICU, of whom 210 (81%) had a serum calcium (Ca++) measured. Of these 210 patients, 135 (64%) were hypocalcemic (serum Ca++< 8.5 mg/dl) and 75 (36%) were normocalcemic. Serum albumin concentration was++concentration may have been normal in many of these patients. On the other hand, 32% of the hypocalcemic patients were alkalotic (pH ≥7.45) which indicates that ionized Ca++levels may have been low because Ca++binding to protein increases with alkalosis. Gastrointestinal bleeders and postabdominal surgery patients were more likely to have low total serum Ca++whereas cardiac and neurosurgical patients were more likely to have a normal total serum Ca++(p< 0.05). Ionized Ca++was calculated in 36 of the normocalcemic and 80 of the hypocalcemic patients. The hypocalcemic group had significantly lower ionized Ca++levels when compared to those of the normocalcemic group (p< 0.001). Patients with low serum Ca++values spent a longer time in the ICU (p< 0.01), had an increased incidence of renal failure and sepsis (p< 0.01), had an increased mortality rate (p< 0.002), and received a greater number of blood transfusions (p< 0.001) than patients in the normocalcemic group. It is concluded that: (1) hypocalcemia is a frequent finding in critically ill patients; (2) determining ionized Ca++levels is useful because many ICU patients have alterations in both arterial pH and serum albumin levels; (3) hypoalbuminemia, sepsis, red cell transfusions, and renal failure are predisposing factors for hypocalcemia; and (4) hypocalcemic patients do less well clinically than normocalcemic patients.