The association of hypoglycemia and microphallus in the male neonate is presumptive evidence of congenital hypopituitarism. This was observed in four male infants with normal birth weight and length, optic discs, and intelligence, and without gross central nervous system malformations. Plasma and urinary cortisol values were low. Stimulation with metyrapone and insulin hypoglycemia failed to elicit a rise in plasma corticoids, but multiple doses of ACTH evoked a response. Growth hormone responses to arginine, insulin, sleep, L-dopa, and glucagon were uniformly less than 2.5 ng/ml. In three patients, however, length remained within 2 SD of the mean until two years of age; in one, there was a sharp decrease in growth by three months. Two patients had low plasma TSH and thyroxine concentrations within the first month of life. In the other two patients, whose thyroxine levels were measurable, intravenous administration of thyrotropin-releasing factor evoked a normal rise in plasma TSH; serum thyroxine decreased into the hypothyroid range in one after GH therapy was initiated. Plasma prolactin was normal in the first two patients receiving thyroxine replacement therapy. The other two patients had elevated baseline prolactin levels and had an augmented rise in plasma prolactin after administration of TRF. Human chorionic gonadotropin induced a 10- to 15-fold rise in plasma testosterone in the two patients tested. The changes in plasma FSH and LH after luteinizing hormone-releasing factor were either low or in the prepubertal range. In three patients, treated with testosterone enanthate intramuscularly, phallic growth occurred. In addition, all three had a transient increase in height but no acceleration of skeletal maturation. The data suggest a deficiency of hypothalamic hypophysiotropic hormones rather than a primary pituitary defect. Early recognition of this syndrome complex is critical for prompt treatment of the life-threatening cortisol deficiency. The diagnosis is more difficult in affected females because their external genitals are normal. The microphallus is a remediable manifestation of hypopituitarism.