Forty-nine patients with primary hyperparathyroidism were examined preoperatively and three months after parathyroid operation for their serum vitamin D metabolites and routine laboratory samples related to calcium metabolism. The preoperative serum 24,25-dihydroxyvitamin D level, mean (SE) was 1.86 (0.22) nmol/l and the postoperative level 5.35 (0.63) nmol/l, the difference being highly significant (P < 0.001). Serum 1,25-dihydroxyvitamin D levels fell significantly (P <0.001) from a preoperative level of 175.5 (17.9) pmol/l to 102.8 (10.1) pmol/l postoperatively. The preoperative 25-hydroxyvitamin D level did not change significantly after surgery. The preoperative serum 24,25-dihydroxyvitamin D level was very low, especially in patients with bone disease while serum parathormone was significantly higher than in patients without bone disease.