The response of GH to acute administration of GH-releasing hormone (GHRH) was evaluated in 19 patients with thalassemia major and 8 normal children. In 13 of the 19 patients, GHRH induced a definite increase (greater than 5 ng/ml) in plasma GH levels, with peaks occurring 5-45 min postinjection. In 6 patients there was little or no GH rise after GHRH treatment. Overall, the mean GH response to GHRH of patients with thalassemia was lower than that of normal children. These data indicate that in thalassemia major, in addition to the described defect at the hepatic GH receptor or postreceptor level which impedes generation of somatomedins, there may be a marked impairment in somatotroph function. In one patient in whom the GH response to GHRH was superimposable on that of normal subjects, there was a blunted GH response to insulin hypoglycemia. This finding indicates that functional damage in hypothalamic structures for GH control can also occur in thalassemic patients.