150 PHPT patients were randomized six weeks after surgery, 75 into each group, to 1 year of oral treatment with either calcium carbonate 1 g (500 mg twice daily) alone or calcium carbonate 1 g combined with cholecalciferol 1600 IU (800 IU twice daily). Exclusion criteria were age under 18, manifest osteoporosis at PHPTdiagnosis, persistent hypercalcemia after surgery, postoperative hypocalcemia requiring vitamin D treatment, glomerular filtration rate (GFR) below 40 ml/min, pregnancy, breast feeding, or logistical difficulties, for example living far from the hospital. Included patients had to withdraw any current supplementation with calcium and vitamin D during the study period. All tablets were identical in appearance, the tins were numbered and randomization followed a list compiled by an independent clinical research support organization. The time for randomization was chosen to make sure that the PHPT patients were cured before starting the study medication. Nearly three-quarters of the patients were randomized within 6 weeks after PTX.The primary end point was the change in PTH after PTX and treatment with the study medication. Secondary end points were vitamin D levels, insulin resistance, blood pressure and other cardiovascular risk factors, biochemical markers of bone turnover, bone mineral density and self estimated health related quality of life (SF- 36).