Increased levels of intact parathyroid hormone (PTH) have been documented after surgery for primary hyperparathyroidism (pHPT) despite normocalcemia. The pathogenesis remains to be elucidated. Seventeen consecutive patients operated on for solitary parathyroid adenoma were investigated before and at 8 weeks and 1 year after surgery with serum levels of intact PTH, biochemical variables known to reflect PTH activity, and bone mineral content (BMC). In addition, an oral calcium loading test was performed 8 weeks after the operation. All patients had low or normal serum calcium levels during follow-up. Eight weeks after operation six patients (35%) had an increased serum PTH level. These patients (group I) preoperatively had higher serum levels of PTH and alkaline phosphatase than patients with normal PTH levels (group II). They also had lower BMC and larger parathyroid adenomas. They did not differ in renal function. At 8 weeks after operation group I showed higher mean serum levels of osteocalcin and propeptide of type I procollagen but lower urinary calcium excretion. In contrast to patients in group II, they also showed a lower calciuric response and a trend to a lower calcemic response during the oral calcium load. The two groups showed similar parathyroid sensitivity for calcium. Patients in group I demonstrated a significant increase in BMC the first year after the operation. Increased serum PTH 8 weeks after surgery for sporadic parathyroid adenoma was not due to persistent pHPT or impaired renal function. Instead, the results imply there is diminished calcium absorption and increased bone turnover with cortical bone remineralization.