Hypothalamic-pituitary function was studied in 45 patients with idiopathic GH deficiency (GHD), 33 of whom had pituitary abnormalities on magnetic resonance imaging: pituitary hypoplasia, undescended stalk and ectopia of the posterior lobe in 8 patients with isolated GHD (IGHD) (group I) and in 12 patients with multiple pituitary hormone deficiency (MPHD) (group II); isolated pituitary hypoplasia in 13 patients with IGHD (group III); no evidence of pituitary abnormalities in the remaining 12 patients with IGHD (group IV). Sellar and pituitary volumes were significantly lower in groups I, II, and III than in group IV (P less than 0.001). No significant differences were observed between group I and group II in the GH response to GHRH1-44 expressed both as peak serum GH and area under the curve. Mean GH peak in group III and IV was significantly higher than that in group I (P less than 0.005) and II (P less than 0.001), as were the mean AUC (P less than 0.005), suggesting hypothalamic defect. Delayed peak serum TSH after TRH was found in all patients of group II, and overt hypothyroidism in 11 of them. Furthermore, basal hyperprolactinemia was present in 6 patients and adrenal insufficiency in 7 cases of group II. Finally, a reduced response of FSH to GnRH was observed in all these patients (P less than 0.005 vs. each of the other groups), and clinical hypogonadism was present in all of them. We suggest that: 1) A high incidence of pituitary abnormalities seems to be present in idiopathic GHD patients; 2) Pituitary hormone deficiencies are more dependent on the type of the hypothalamic-pituitary abnormality than on the size of the pituitary per se: the association of pituitary hypoplasia, undescended stalk and ectopia of the posterior lobe should possibly be considered a distinct entity reflecting an early abnormality in hypothalamic development; 3) The majority of patients with IGHD or MPHD probably have a primary hypothalamic releasing hormone deficiency even if pituitary hypoplasia is associated; 4) Magnetic resonance imaging may have a role in the diagnosis and prognosis of patients with GHD through differentiation between patients who are at risk for developing MPHD vs. those who are candidates for having a persistently isolated GHD.