Several studies have documented elevated parathyroid hormone (PTH) levels after seemingly successful exploration for primary hyperparathyroidism (pHPT). It is not known if this is a transient phenomenon after pHPT surgery or if it predisposes to recurrent disease. A series of 99 consecutive patients with pHPT who had solitary parathyroid adenomas were followed for 5 years. Serum levels of PTH and biochemical variables reflecting PTH activity were measured before operation, at 8 weeks postoperatively, and then yearly for 5 years. All patients were normocalcemic after exploration. At 8 weeks after operation 28% of the patients had elevated serum PTH levels; at 5 years this figure decreased to 16%. During the 5-year follow-up one group of patients normalized their PTH levels, another group's PTH levels fluctuated, and still another group had consistently normal PTH levels. Patients with fluctuating PTH levels had increased levels of serum calcium and phosphate. Some of these patients (15%) showed signs of impaired renal function. Two patients with consistently elevated PTH levels showed signs of mild renal dysfunction, and one of them developed recurrent HPT. Elevated PTH levels after successful parathyroid surgery is not a transient phenomenon. An increased risk for recurrent disease is postulated for some of the patients who do not normalize their PTH levels postoperatively, and long-term surveillance of these patients is suggested.