There is evidence for sick very preterm infants (VPI) having inadequate low random serum cortisol (F) levels[1]. However there are no well established reference ranges for F in VPI < 30 weeks of gestational age (GA) because of the high morbidity in these patients. The aim of this prospective study was to measure F in well VPI < 30 weeks of GA during the first two weeks. Methods: After birth VPI were randomly assigned to one day of blood sampling during the first 14 days. The sample was taken between 4.00 and 8.00 a.m by venous puncture. F was measured by radio immuno assay. The VPI had no family history of adrenal illness, no congenital anomalies, no surgery and no postnatal corticosteroid therapy before blood sampling. The following exclusion criteria were valid during 48 hours before and 24 hours after withdrawing the blood sample: surfactant administration, arterial hypotension, acute or uncontrolled infection, ventricular hemorrhage>II°, blood glucose < 40 mg/dl, exchange transfusion, stress by any kind of examination or nursing for at least 4 hours before blood sampling.Results: 41 VPI (23 female, 18 male) were included in the study. Only one VPI was ventilated at the time of blood sampling. GA was 27.7 weeks(24.7 - 29.9) (median, range) and birth weight was 960 g (620 - 1580, 7 small for gestational age). 32/41 had received prenatal steroids. Median F was 8.6μg/dl, range 2.1 - 74.3 μg/dl, 25th/75th percentiles 5.5/12.2 11/41 of F were < 5 μg/dl. If two outliers (F=44.7/74.3) were excluded, only weak negative correlations were found between F and postnatal age (r = -0.18), gestational age (r = -0.24) or birth weight (r = -0.30). Discussion: We present first reference values for serum F in a strictly defined population of well VPI < 30 weeks of GA. Compared to previously published F values in a more mature population[2] the presented values are higher (median 8.6 vs 5.1 μg/dl). 27% of our subjects not revealing clinical signs of adrenal insufficiency showed F levels below 5 μg/dl, a threshold below which adrenal insufficiency is assumed[3].