The carbohydrate-deficient glycoprotein (CDG) syndrome type 1 is a genetic multisystem disorder, characterized by hypoglycosylation of glycoproteins and presenting with neurologic impairment. In 12 girls and 14 boys, we confirmed the diagnosis of CDG syndrome type 1 by immune-isoelectric focusing of serum sialotransferrins, and we examined the endocrine status singly or sequentially, including a 16-y follow-up of the index cases, a pair of monozygotic girls. Serum FSH levels were normal in newborns and prepubertal children, but elevated in female toddlers and teenagers, as well as in adolescent males. Serum LH concentrations displayed an analogous age-dependent pattern. In adolescent girls, serum estradiol remained low. FSH bioactivity was low normal, as was the bioactive/immunoreactive FSH ratio. However, exogenous gonadotropins evoked an estradiol response and induced ovarian follicular growth. Male patients virilized at puberty; however, testicular volume was subnormal. The thyroid axis was hallmarked by thyroid-binding globin deficiency and, during infancy, increased serum TSH concentrations. A subgroup of female patients presented hypersomatotropism and/or hyperprolactinemia. During adolescence, the index cases responded to glucagon with normal glycemic, but exaggerated insulin and paradoxically augmented growth hormone responses. The hypothalamo-pituitary area appeared intact on magnetic resonance imaging. Circulating IGF-1 levels were in the lower normal range and transcortin concentrations decreased. In conclusion, a study of endocrine aspects of a major glycosylation disorder revealed an age-dependent constellation, including hypergonadotropic hypogonadism with deficient FSH rather than LH action; transient hyperthyrotropinemia; inconsistent hyperprolactinemia; hyperglycemia-induced growth hormone release; deficiencies of hormone-binding glycoproteins and possibly decreased insulin sensitivity, thus pointing to the importance of glycoprotein glycosylation for pediatric endocrinology.