Abstract Primary hyperparathyroidism (PHP) is associated with impaired GH secretion. Whether this effect is due to hypercalcemia or to increased serum PTH concentration is unclear. However, patients with familial hypocalciuric hypercalcemia (FHH), who have normal PTH and increased serum calcium concentrations, also have an impaired GH secretion, suggesting that calcium rather than PTH is responsable for this effect on GH secretion. To further investigate this issue, 10 consecutive patients with secondary hyperparathyroidism (SHP) due to vitamin D deficiency were evaluated by the GH response to GHRH+arginine (Arg) test. A group of 60 consecutive untreated PHP patients served as controls. Mean GH response to GHRH+Arg test was 15.8+/-14 microg/l and 37.5+/-16 microg/l (p<0.001) in PHP and in SHP patients, respectively. Forty-two out of 60 (70%) PHP patients had a suppressed or blunted GH response, whereas all SHP patients had normal GH response. The results of the present study confirm and extend our previous observations that PHP is associated with an impaired GH secretion in the majority of cases, and indicate that SHP patients have no abnormality of GH secretion. Thus, hypercalcemia rather than increased serum PTH is responsible for the abnormality of GH secretion.