Serum levels of insulin-like growth factor I (IGF-I), IGF-binding protein-3 (IGFBP-3), the acid-labile subunit (ALS), insulin, and IGFBP-1 were evaluated as indicators of body composition during GH replacement therapy in 20 GH-deficient patients (9 women and 11 men), aged 22-57 yr, with IGF-I levels below -2 SD. The mean GH dose was 0.128 +/- 0.003 IU/kg.week during the first month and thereafter 0.23 +/- 0.01 IU/kg.week, divided into daily doses (0.7-4.3 IU/day). Serum levels of IGF-I, ALS, and IGFBP-3 above the normal range were reached in seven, five, and three subjects, respectively, after 12 months of GH therapy. IGF-I and ALS levels, but not IGFBP-3 levels, correlated with the total daily GH dose (r = 0.676; P = 0.001 and r = 0.631; P = 0.003). The mean increase in lean body mass (LBM) measured by dual energy x-ray absorptiometry was 3.0 +/- 0.5 kg (P < 0.001). At 12 months, the LBM values were significantly correlated to the IGF-I levels (r = 0.718; P < 0.001), but not to ALS or IGFBP-3 levels. No correlation was found before therapy, and the increase in LBM at 12 months correlated with the IGF-I increase (r = 0.514; P = 0.029) only after exclusion of two nonresponders. Both before and during therapy, LBM was inversely related to IGFBP-1 (r = -0.715; P < 0.004 at 12 months). None of the GH-induced proteins could be used as indicators of body fat changes. In conclusion, both IGF-I and ALS can be used as indicators to avoid GH excess during replacement therapy, but only IGF-I relates to changes in LBM.