Background: Abnormalities in calcium and vitamin D metabolism are observed early after gastric bypass, whereas clinical or biochemical evidence of metabolic bone disease might not be detected until many years after the procedure. The aim of the present study was to evaluate the impact of bariatric surgery on bone metabolism determined on the basis of postoperative laboratory changes in calcium, phosphorus, magnesium, alkaline phosphatase and parathormone (PTH) levels. Methods: 110 patients submitted to Roux-en-Y gastric bypass (RYGBP) were followed after surgery, and the following parameters were determined: intact PTH molecule (PTHi; chemiluminescence), alkaline phosphatase (colorimetric method), ionic calcium (selective electrode), phosphorus and magnesium (colorimetric method). Results: Elevated serum PTHi levels were observed in 29% of the patients and hypocalcemia in 0.9% from the 3rd postoperative month and afterwards (3 to 80 months after surgery). Conclusion: There is a need for careful evaluation of bone metabolism and for routine calcium replacement after RYGBP.