This prospective study aims to investigate the factors that influence the human CRH (hCRH) test and to provide reference ranges for plasma corticotropin (ACTH) and serum cortisol concentrations of the stimulation test in preterm, very low birth weight (VLBW) infants. Two hundred twenty-six hCRH tests were performed on 137 VLBW infants at d 7 and 14 of life. Plasma ACTH did not differ significantly between infants whose mothers did not receive antenatal corticosteroids (group 1) and those whose mothers received one or two doses (group 2) or more than two doses (group 3) of the drug. However, plasma ACTH levels at d 7 were found to be significantly higher in infants with severe lung disease who required intermittent positive-pressure ventilation (IPPV) or high-frequency oscillation ventilation (HFOV), compared with those who had milder pulmonary disease and did not require mechanical ventilation or needed only continuous positive airway pressure (CPAP) support (P < 0.011). A significantly higher rate of increase in plasma ACTH concentration at d 7 was also observed in infants whose mothers suffered from antepartum hemorrhage (P < 0.016). In contrast, infants in group 2 had significantly lower serum cortisol, compared with group 1 infants (P < 0.05), whereas group 3 infants did not have serum cortisol levels significantly different from those of patients in group 1 or 2. Significant positive correlation between serum cortisol at d 7 and the time interval between the last dose of antenatal dexamethasone and delivery was also observed in group 3 infants (r > 0.33, P < 0.045). In addition, infants who required IPPV or HFOV had significantly lower serum cortisol at d 7 (P < 0.0001), but this pattern of cortisol response was reversed on d 14, with infants requiring IPPV or HFOV having significantly higher serum cortisol (P < 0.036). The reference ranges for plasma ACTH and serum cortisol concentrations of the hCRH test at d 7 and 14 were also provided for group 1 and group 2 infants. This study demonstrates that even one or two doses of antenatal corticosteroids cause adrenal suppression in VLBW infants. Maternal antepartum hemorrhage also influences the pituitary response of preterm newborns in the first week of life. The change in the pattern of cortisol response in sick ventilated (IPPV or HFOV) infants during the first 2 wk of life suggests that a proportion of preterm infants may have inadequate adrenal response to stress in early postnatal life, but it is likely that rapid adaptation of the hypothalamic-pituitary-adrenal axis results in enhanced and more appropriate cortisol response by d 14. The percentile distribution of plasma ACTH and serum cortisol responses provides useful statistical reference data for interpretation of the hCRH test in VLBW infants and may also assist in facilitating the use of corticosteroids replacement therapy in cases with clinical manifestations suggestive of adrenal insufficiency.