Bone Gla protein (BGP) is a vitamin K-dependent protein which is a marker of bone turnover. To determine whether serum BGP is a useful indicator for parathyroidectomy in patients with secondary hyperparathyroidism, we measured serum BGP levels. Thirty-seven patients with secondary hyperparathyroidism who were followed up for more than 1 year after parathyroidectomy were studied. All patients underwent total parathyroidectomy and autotransplantation. Controls were 46 patients who were treated by chronic hemodialysis for more than 3 years. Serum BGP levels (normal: less than 6.5 ng/ml) were markedly increased in 37 patients with parathyroidectomy, ranging from 4.2 ng/ml to 645 ng/ml, with a mean value of 278.8 +/- 159.8 ng/ml (mean +/- standard deviation) versus 65.0 +/- 85.2 ng/ml in the 46 controls (p less than 0.001). Patients with a high BGP level had severe bone and joint pain. Serum BGP in patients with parathyroidectomy was significantly correlated with serum alkaline phosphatase and mPTH (p less than 0.001 for both). The total weight of resected parathyroid tumors was 2,152 +/- 1,368 mg, and tumor weights ranged from 200 mg to 5,600 mg. There was a highly significant correlation between BGP level and tumor weight (r = 0.656, p less than 0.001). The 2 patients who showed BGP levels below 10 ng/ml had tumor weights of only 470 mg and 240 mg, respectively, and revealed no improvement of pain postoperatively, although their mPTH levels were increased. These results suggested that BGP measurement is a sensitive method for detecting increased bone turnover and is possibly useful as an indicator for parathyroidectomy in patients with secondary hyperparathyroidism.