This study evaluates the effect of the spontaneous pubertal increase in sex steroids on GH secretion in GH-deficient patients. Fifteen patients (10 boys, 5 girls) with idiopathic isolated GH deficiency diagnosed before puberty (GH peak < 8 µg/l after 2 arginine insulin stimulation tests) were reevaluated for their GH secretion using the same test after completion of their hGH therapy and puberty. Their ages at diagnosis and at the last evaluation were 8.2 ± 0.7 (SE) (range 4.9-14.9) and 17.8 ± 0.3 years (15-23), respectively. The data at diagnosis and at last evaluation showed that (1) the mean height increased from -4 ± 0.3 to -2.5 ± 0.3 SD (p < 0.01), (2) the mean GH peak increased from 4.4 ± 0.3 (1.6-8) to 7.6 ± 0.8 µg/l (2-13.2, p < 0.01); at the last evaluation, 8/15 patients had GH peak > 8 µg/l and (3) the mean plasma insulin-like growth factor I increased from 0.28 ± 0.05 to 0.42 ± 0.03 U/ml (n = 6, p < 0.05). The mean increase in the GH peak was 3.2 µg/l (-3 to 10.6). It was negatively correlated with the degree of growth retardation at diagnosis (r = -0.74, p < 0.005). We conclude that the increase in the GH peak at puberty in patients with GH deficiency reflects the severity of GH deficiency and that a corrective factor of the cutoff number is necessary for the diagnosis of GH deficiency in puberty.