Serum insulin and C-peptide response to an oral glucose tolerance test (OGTT) and serum growth hormone (GH) response to an intravenous insulin tolerance test (IVITT) were investigated in six non-obese patients, with normal glucose tolerance, affected by diffuse idiopathic skeletal hyperostosis (DISH). Basal serum insulin, C-peptide and GH values were similar in DISH patients and in controls. After OGTT, insulin and C-peptide values were not significantly different in the two groups at any time interval. In contrast, after IVITT, a significant increase in GH concentrations was noted at 30 and 45 minutes in DISH patients when compared to controls. Growth hormone either acting alone or through somatomedin intermediaries results in new bone growth in acromegaly, and the same may be true for DISH. Recent reports suggest that GH promotes tissue growth by stimulating precursor cells in various tissues including cartilage and bone. The increased GH response to IVITT corroborates the hypothesis that GH may act as a bone growth-promoting factor in DISH.