OBJECTIVE The recognition of the syndrome of adult GH deficiency suggests that young GH deficient adults, deprived of GH replacement at completion of linear growth, may suffer effects of GH deficiency. We assessed GH reserve in young adults previously diagnosed as having idiopathic GH insufficiency, who were treated with hGH replacement (14 IU/m2/week) in childhood.DESIGN Eight patients (7 males, 1 female) diagnosed as having GH insufficiency by insulin tolerance test (ITT) in childhood (ages 8.5–15.6 years) were retested by ITT at completion of linear growth (ages 15.1–19.6 years), 3 months after discontinuation of hGH therapy.MEASUREMENTS GH reserve was measured during ITT at diagnosis and at retesting. Height velocity (HV) and HV SDS were calculated before and during GH therapy.RESULTS At diagnosis, the mean peak GH response to ITT was 10.5 ± 2.0 mU/l (range 7.7–13.6). At retesting, mean GH was 52.4 ± 33.2 mU/l (range 10.4–100), 7/8 subjects having peak GH levels greater than 15 mU/l. During hGH therapy mean HV increased from 4.0 ± 1.5 cm/year at diagnosis to 7.3 ± 1.9 cm/year during the 1st year (P = 0.004) and 6.9 ± 2.3 cm/year during the 2nd year (P = 0.02). Mean HVSDS increased from −1.6 ± 2.1 at diagnosis to 3.1 ± 2.9 during the 1st year (P = 0.004) and 2.2 ± 4.2 during the 2nd year (P = 0.05, NS) of treatment.CONCLUSIONS Seven out of 8 children diagnosed as having idiopathic GH insufficiency had normal GH secretion at completion of linear growth. Children with GH insufficiency cannot be assumed to become GH deficient adults and should not continue on GH therapy into adult life without reinvestigation. All who were GH insufficient children should be retested at completion of linear growth to ide