Sixty-five patients (22 boys and 43 girls) presenting with familial tall stature were investigated with regard to growth hormone (GH) secretion, both physiological and after stimulation with thyrotropin releasing hormone (TRH) and growth hormone releasing hormone (GHRH). Plasma insulin-like growth factor-I (IGF-I) was also measured. Two groups of patients were distinguished according to their physiological secretion of GH: a high secretory group (n = 49) with a mean 24 h integrated concentration of GH (IC-GH) of 5.4 +/- 2.3 micrograms/l per minute and a large number of peaks (5.1 +/- 1.6 in 24 h), and a low secretory group (n = 16) with a mean 24 h IC-GH of 2.1 +/- 0.5 micrograms/l per minute and few peaks (3.3 +/- 1.3 in 24 h). Plasma IGF-I levels and GH peak values after the TRH test were significantly higher in the high secretory group. These results indicate that familial tall stature is the consequence either of hypersecretion of GH or of hypersensitivity to this hormone (IGF-I levels being normal in spite of low GH levels).