OBJECTIVE: To evaluate the differences in adrenal function between very low birth weight (VLBW) infants from singleton and multiple pregnancies. DESIGN AND METHODS: Forty infants of birth weights less than 1500 g underwent an ACTH test. Thirty infants born from singleton pregnancies (singleton group) and ten born from multiple pregnancies (multiple group) were enrolled. A baseline blood sample was drawn for cortisol measurement and thereafter serum cortisol was measured 1 and 2 h after an i.v. injection of ACTH. RESULTS: In multiple pregnancies, the median basal cortisol level of the infants was significantly lower than that in the singletons. The median cortisol level at 1 and 2 h after administration of ACTH was significantly lower in infants from multiple gestations than in singletons. Of infants from the multiple gestation group six, and of the singleton infants 12, had baseline cortisol levels lower than the reference values (P=0.48). One hour after ACTH stimulation all multiple and 53% of the singleton group infants showed a subnormal (<500 nmol/l, P=0.007) cortisol response. Two hours after ACTH, nine multiple group patients and 43% of the singletons had subnormal (<500 nmol/l, P=0.01) stimulated cortisol levels. CONCLUSIONS: We have concluded that VLBW infants from multiple gestations seem to be at an increased risk of insufficient postnatal adrenocortical function. In the future, specific attention should be paid to evaluate further newborn infants from multiple pregnancies with regard to a possible benefit of hydrocortisone substitution in stressful clinical situations.