Spontaneous growth and growth responses to GH therapy vary considerably among girls with Turner's syndrome. In an attempt to clarify this variability, we assessed growth parameters, 24-h GH profiles, arginine-stimulated serum GH levels, and plasma insulin-like growth factor-I (IGF-I) concentrations in a group of 41 girls with Turner's syndrome with a mean (+/- SD) age of 13 +/- 3 yr (range, 6.7-18.9). We subsequently treated all girls with biosynthetic GH (24 IU/m2 x week) and documented the growth response after 1 yr of therapy. GH profiles were analyzed according to Pulsar and Cluster, and GH secretion rates were calculated by waveform-independent deconvolution (Pulse). Factor analysis selected the mean 24-h GH secretion rate and number of GH peaks according to Cluster and Pulse as the principal GH profile variables to be used for further analysis. The mean (+/- SD) daily pituitary GH secretion rate was 127 +/- 47 micrograms/L.24 h (range, 37-232). The GH secretion rate correlated inversely with body mass index (r = -0.45; P < 0.01; n = 41). There was no relationship between the GH secretion rate and the growth parameters before or after GH therapy. However, the number of GH peaks (Pulse) correlated negatively with baseline height velocity (r = -0.53; P = 0.03) and was a positive predictor for height velocity increment during the first year of GH therapy (r = 0.71, P = 0.001). The mean (+/- SD) IGF-I level was 217 +/- 91 ng/mL (range, 87-413). There was no relationship between GH secretion rate or growth parameters and IGF-I. However, the number of GH peaks correlated negatively with IGF-I (r = -0.49; P = 0.04; n = 17). We conclude that an elevated spontaneous GH pulse frequency pattern is associated with relatively low IGF-I levels and slow baseline growth in girls with Turner's syndrome and that girls with such a pulse pattern may benefit most from exogenous GH therapy.