Objective: Lithium carbonate may affect calcium metabolism and alter parathyroid physiology. The purpose of this study was to test the hypothesis that long-term lithium treatment can induce alterations in serum total calcium and biologically active (intact) parathormone (PTH) levels in euthymic bipolar patients. Method: Serum total calcium levels were measured with standard autoanalyzer techniques, and intact PTH (iPTH) levels were assessed by RIA in 10 lithium-naive (mean age±SD: 34.50±4.85) and 15 long-term lithium treated (mean age±SD: 34.46±10.16) bipolar patients. Results: Both serum total calcium and iPTH levels were found significantly higher in the lithium treated group compared to lithium-naïve group and there was a positive correlation between these two biochemical variables. Conclusion: These data demonstrate that long-term lithium treatment may alter serum calcium and PTH activity.