ABSTRACT.Integrated 12‐hour growth hormone secretion studies, peak growth hormone response to clonidine provocation. Somatomedin‐C levels, T‐4 and TSH levels were studied in six growth‐retarded children with the Prader‐Willi syndrome, of whom five had a 15 q‐karyotype. Only one of the subjects was obese. All showed abnormally low growth hormone secretion. None achieved a nocturnal peak above 10 μg/l, none had a mean nocturnal level over 1.8, and none showed a level above 8 μg/l after clonidine provocation. These findings contrasted with normal TSH in all and normal T‐4 in five. These findings suggest that the poor linear growth in the Prader‐Willi syndrome is caused by a true deficiency of growth hormone secretion, and that the low growth hormone levels observed in such cases are not an a