The response to an oral calcium load test was assessed in 17 hypercalciuric nephrolithiasis patients who presented elevated parathyroid hormone (PTH) irrespective of the ionized calcium (sCa 2+ ) levels. Blood samples were collected at baseline (0 min) and at 60 and 180 min after 1 g calcium load for serum PTH, total calcium, sCa 2+ , and 1.25(OH) 2 D 3 determinations. According to the sCa 2+ level at baseline, patients were classified as normocalcemic (N = 9) or hypercalcemic (N = 8). Six healthy subjects were also evaluated as controls. Bone mineral density was reduced in 14/17 patients. In the normocalcemic group, mean PTH levels at 0, 60 and 180 min (95 ± 76, 56 ± 40, 57 ± 45 pg/ml, respectively) did not differ from the hypercalcemic group (130 ± 75, 68 ± 35, 80 ± 33 pg/ml) but were significantly higher compared to healthy subjects despite a similar elevation in sCa 2+ after 60 and 180 min vs baseline in all 3 groups. Mean total calcium and 1.25(OH) 2 D 3 were similar in the 3 groups. Additionally, we observed that 5 of 9 normocalcemic patients presented a significantly higher concentration-time curve for serum PTH (AUC 0',60',180' ) than the other 4 patients and the healthy subjects, suggesting a primary parathyroid dysfunction. These data suggest that the individual response to an oral calcium load test may be a valuable dynamic tool to disclose a subtle primary hyperparathyroidism in patients with high PTH and fluctuating sCa 2+ levels, avoiding repeated measurements of both parameters.