Seventeen patients with idiopathic growth hormone deficiency (GHD) were divided into two groups: one has no perinatal abnormalities (group A, n = 7) and the other has perinatal abnormalities, i.e. breech delivery and asphyxia (group B, n = 10). To see whether there are any differences in hypothalamo-pituitary dysfunctions in the two groups, the pituitary growth hormone (GH) reserve was examined. After 100 micrograms of synthetic growth hormone releasing hormone (GHRH) injection, group A showed a much higher peak values compared to group B (mean +/- S.E.: 16.1 +/- 3.5 ng/ml vs. 3.6 +/- 0.7 ng/ml, p less than 0.01), although there were no differences in their baseline GH values. In addition, plasma GH responses to arginine and L-dopa, which were performed at the diagnosis of GHD, were also greater in group A than group B (mean peak value: arginine, 3.4 +/- 0.5 ng/ml vs 1.8 +/- 0.5 ng/ml, p less than 0.05; L-dopa, 3.2 +/- 0.7 ng/ml vs. 1.3 +/- 0.2 ng/ml, p less than 0.01). There were no significant differences in the bone ages in the two groups, but bone age to chronological age ratio and pubertal development were significantly lower in group B. High frequency of primipara was observed in group B (7/10) compared to group A (2/7). These results indicate that pituitary GH reserve is much impaired in GHD with abnormal delivery compared to that without abnormal delivery, probably depending on the irreversible hypothalamo-pituitary damages due to prolonged anoxic state during the delivery. Especially, such risks seems to be high when cases of breech presentation are delivered from primipara mothers.