QT prolongation induced by antipsychotics has been reported to be a determinant for the development of torsade de pointes and sudden death. However, the effect of lithium on QT interval has not been fully clarified. The aim of the present study was to elucidate the relationship between serum lithium concentration and QT interval in patients treated with lithium. We examined serum lithium concentrations and electrocardiographic features in 39 inpatients with bipolar affective disorder or schizophrenia. The longest QT interval in the 12 electrocardiographic leads was measured using GE Marquette QT guard System Software, and Bazett formula was used for heart rate correction. The longest QTc was positively correlated with lithium concentration ( r = 0.46, P = .003). Multiple regression analysis revealed that sex (female, P = .037), lower serum K + concentration ( P = .029), and especially, higher serum lithium concentration ( P = .009) were determinants for the prolongation of the QTc.