The detection of pituitary stalk interruption syndrome (PSIS) by magnetic resonance imaging is a diagnostic marker of permanent GH deficiency (GHD), but the pathogenesis of PSIS is unknown. Fifty-one patients (27 males) with GHD and PSIS were classified according to whether the GHD was isolated (group 1, 16 cases) or associated with other anterior pituitary abnormalities (group 2, 35 cases). The 2 groups had similar characteristics (frequencies of perinatal abnormalities, ages at occurrence of first signs and at diagnosis, height, GH peak response to stimuli other than GHRH), but associated malformations were less frequent in group 1 (12%) than in group 2 (54%; P < 0.01), hypoglycemia occurred in 25% of group 1 and 70% of group 2 (P < 0.01), and the GH peak response to GHRH was less than 10 micrograms/L in 0% of group 1 (4 cases evaluated) and 57% of group 2 (21 cases; P < 0.05). Thirty-one cases (61%; 25 from group 2) had features suggesting an antenatal origin: familial form (4 cases), microphallus (10 boys), and/or associated malformations (50%; 21 cases). Twenty-seven cases (53%, 22 from group 2) had features suggesting a hypothalamic origin. The three group 1 patients with a GH peak of 1 microgram/L or less had no large GH-N gene deletion. One familial form had no linkage between the GHD phenotype and abnormal GH-N locus, and the only mutation described to date in the GHRH receptor gene was absent. The two patients with low plasma PRL levels had no Pit-1 gene abnormality. Thus, most of the patients with GHD associated with multiple anterior pituitary abnormalities and PSIS have features suggesting an antenatal origin. The GH-N, GHRH receptor, and Pit-1 genes do not seem to be implicated in PSIS.