In 41 girls with Turner syndrome, the growth hormone (GH) peak values during stimulation tests and parameters of spontaneous nocturnal GH secretion were studied and compared with respect to different karyotypes, short-term growth response to GH therapy, and final height. 22.0% of the girls tested had a subnormal (peak < 11 ng/ml) and 9.7% a pathological (< 7 ng/ml) GH response. The spontaneous GH secretion showed a good correlation with the data of the provocation tests, providing no further information regarding GH capacity. Short-term growth response to GH treatment could not be predicted by any of the investigated parameters. Although patients with isochromosomes had frequent subnormal GH tests, their growth response to GH treatment after 1 year was comparable to that of girls with XO karyotype and mosaicism. In 18 patients who had reached final height, the height gain during treatment (calculated as final height minus projected adult height) was not different among patients with normal, subnormal, or pathological GH tests. In contrast, final height minus projected adult height in 4 girls with isochromosomes was 15.7 +/- 5.1 versus 7.6 +/- 3.3 cm in 14 patients with other karyotypes (p < 0.01). These girls had a more pronounced bone age delay (3.3 +/- 0.3 vs. 1.8 +/- 1.2 years) at the start of therapy and thus a better growth potential. We conclude that short- and long-term growth responses to GH treatment in Turner syndrome could not be predicted by GH testing. Patients with isochromosomes might represent a subpopulation which is more frequently GH deficient and shows a marked bone age delay.