Abstract. Background: Newborn infants have low serum cortisol concentrations due to immature Hypothalamic-PituitaryAdrenal axis. Cortisol is highly protein bound. Hypoproteinemia, often present in sick newborns, can alter the measured total serum cortisol (TC) without affecting free cortisol (FC) concentrations. Objective: Evaluate the relationship between baseline FC and TC in sick newborn infants with refractory hypotension. Methods: Simultaneous baseline FC and TC were measured in neonates with refractory hypotension. TC was measured using radioimmunoassay and FC by LC/MS/MS. The correlation between the concentration pairs was determined using regression. Results: Thirty ﬁve of 36 pairs with patients serum cortisol concentrations were included. One pair was removed due to being a strong outlier. The mean ± SD gestational age and birth weight were 29.3 ± 5.7 weeks and 1.52 ± 1.28 kg, respectively. FC (0.02 to 42.5 mcg/dL) and TC (0.7−33.2 mcg/dL) varied widely. The overall correlation between FC and TC was r = 0.48 (p = 0.003). When analyzed based on TC ≤ or > 10 mcg/dL the correlation weakened (r = 0.38 and r = 0.21, respectively). For infants ≤34 and >34 weeks correlation were similar (r = 0.48 and 0.45, respectively). Conclusions: In sick newborns, correlation between FC and TC is weak. TC measured routinely may not reﬂect the extent of adrenal insufﬁciency.