Reliability of corrected QT intervals (QoTc, QaTc, and QeTc) as indicators of clinical hypercalcemia was assessed in 14 hypercalcemic patients. Hypercalcemia was severe to extreme (serum calcium 14.9 to 22.8 mg/dl) in 11, moderate (13.4 mg/dl) in 1, and mild (12.2 and 11.8 mg/dl) in 2 patients. QT intervals during hypercalcemia were compared with those during normocalcemia either before or after development of hypercalcemia. QeTc interval showed neither significant correlation with serum calcium nor any consistent pattern of change with development of hypercalcemia or normalization of serum calcium. In contrast, QoTc and QaTc intervals shortened with development of hypercalcemia and returned toward normal with normalization of serum calcium in all the patients, and showed significant correlation with serum calcium (QoTc: r = -0.77, p less than 0.001, n = 35; QaTc: r = 0.82, p less than 0.001, n = 35). QaTc was short (less than 0.30 s) in all the ECGs in severe and moderate hypercalcemia and in 2 of the 5 ECGs in mild hypercalcemia. Combination of short QoTc (less than 0.18 s) and short QaTc was found to be highly specific for, and was present in 65% of ECGs, in moderate and severe hypercalcemia. Combination of normal QoTc (greater than 0.18 s) and normal QaTc (greater than 0.30 s) was not observed in moderate or severe hypercalcemia. We conclude that QoTc and QaTc intervals are reliable indicators of clinical hypercalcemia.