Diagnosis of GH deficiency (GHD) has been done traditionally by the combination of auxological data and the results of GH provocation tests. Recently, limitations of GH provocation tests have been advocated. Thus, three extreme subgroups (G-1; normal, G2; severe GHD, G-3; short children with normal GH secretion) were selected in order to show that insulin-like growth factor-I (IGF-I) and IGF binding protein-3 (IGFBP-3) levels reflect GH secretion. In G-1 (n = 52), all had normal IGF parameters. In G-2 (n = 27), all had low IGF parameters. In G-3 (n = 28), all except for one patient had normal IGF parameters. Taken together with the assumption that GH secretion status is continuous from 0 to normal (or to acromegalic), the data on IGF parameters in the three subgroups indicate that they are functional tests for GH secretion. IGF-I and IGFBP-3, together with free IGF-I and acid-labile subunit, may replace GH provocation tests in the diagnosis of GH deficiency during childhood; these functional tests for GH secretion, which show minimal intradaily variation, are more physiological and cost-effective than GH provocation test.