Growth hormone binding proteins, insulin-like growth factor I and insulin-like growth factor binding proteins were determined in 54 children and adolescents affected by type 1 diabetes mellitus (25 prepubertal and 29 pubertal) showing reduced height velocity and the results were compared to those of 104 matched controls. Growth hormone binding proteins were similar in prepubertal and pubertal subjects but were significantly lower in the prepubertal diabetic group than in controls. Insulin-like growth factor I was low both in prepubertal and pubertal diabetic subjects. Insulin-like growth factor binding protein 3 was similar to controls, while insulin-like growth factor binding protein 1 and 2 were always high in diabetic children. Insulin-like growth factor binding protein 4 was high only in the prepubertal diabetic group. In conclusion, a low insulin-like growth factor I in diabetic children seems to depend on a GH receptor and/or a postreceptor defect. A low insulin-like growth factor I together with a normal insulin-like growth factor binding protein 3 and high levels of insulin-like growth factor binding proteins 1, 2 and 4 results in a reduced bioavailability of insulin-like growth factor I to target tissues. This could be a possible contributing factor to the reduced height velocity seen in our diabetic children.