For the National Cooperative Growth Study II substudy, data on spontaneous growth hormone (GH) secretion were collected from 5106 children with short stature. Of these, 2123 with complete 12-hour samples were subsequently enrolled in the NCGS. Compared with NCGS enrollees who were not in the NCGS II substudy, these children were significantly older (11.3 ± 3.3 years vs 9.9 ± 4.2 years), had a higher maximum reported GH level (13.3 ± 10.5 μg/L vs 9.2 ± 8.7 μg/L), and were more likely to be male (71% vs 62%) and pubertal (27.3% vs 21.9%) (p <0.001 for all). Height deficit, bone age delay, and pretreatment growth rates were similar. Children who were classified as having GH deficiency on the basis of their response to standard pharmacologic tests had lower spontaneous GH secretion than those who were classified as having idiopathic short stature, but considerable overlap was seen between the two groups on all indexes of spontaneous GH secretion. This finding suggests that the investigators were using serial sampling studies in examining children with short stature who were not growing well but had “normal” GH responses to standard pharmacologic testing.(J PEDIATR 1996;128:S38-41)