To determine sensitivity (Sn), specificity (Sp), positive and negative predictive values (PPV, NPV) of total and calculated calcium and of QTc and QoTc intervals for the diagnosis of hypocalcemia in critically ill newborns. A neonatal intensive care unit. We included all newborn less than 28 days of age; we excluded those with calcium treatment, hypomagnesemia or congenital heart disease. Serum levels of total calcium, albumin and ionic calcium were measured as well as the QTc and QoTc intervals. Values for Sn, Sp, PPV and NPV were calculated according to cutoff points of the literature and with those of our receptor operating curves (ROC). Ionic calcium was considered the gold standard (hypocalcemia < 1.10 mmol/L). We included 53 newborns; 21/53 (40%) had hypocalcemia. Clinical features were similar among patients with or without hypocalcemia. For total calcium the Sn was 47%; Sp = 87%; PPV = 71% and NPV = 72%. For calculated calcium there values were: 55, 75, 57 and 73; for QoTc: 47, 81, 61 and 70%; and for QTc: 19, 87, 50 and 62% respectively. With the ROC values the Sn for total calcium and QTc was improved. Among critical ill newborns, total and calculated calcium were better in identifying patients with hypocalcemia whereas QTc and QoTc were better in those without hypocalcemia.