Objective To investigate the changes in cortisol, adrenocorticotropic hormones (ACTH) in term infants with critical illness and to evaluate their functional status.Methods One hundred and fifty term infants who were transferred to the hospital within 72 hours after birth were involved (gestational age ≥ 37 weeks).These term infants were evaluated by neonatal critical illness scores (NCIS) at the time of admission, and they were divided into a mild group and a severe group according to the NCIS scores.Serum cortisol and ACTH concentrations were measured at the time of admission on day 7 and day 14 after birth.Results (1) Within 72 hours after birth, the basic serum cortisol concentration of severe illness infants [(283.5 ± 196.0) μg/L] was significantly higher than that of the infants with mild illness[(175.0 ± 186.5) μg/L], and there was a significant difference (t =-3.830, P =0.000).The basic serum cortisol concentration of the term infants with severe illness was higher than that of the term infants with mild illness,but there was no significant difference[7 d:(94.1 ±41.3) μg/L vs (62.5 ±37.9) μg/L,14 d:(68.6 ±47.7)μg/L vs (50.9 ± 38.4) μg/L, all P ＞ 0.05].(2) Among the infants with critical illness, the basic serum cortisol concentration within 72 hours after birth was significantly higher than those of day 7 and day 14 after birth (t--5.994, 2.848;P =0.000,0.036).(3) Among the mild infants, the basic serum cortisol concentration within 72 hours after birth was significantly higher than those of day 7 and day 14 after birth (t =4.691,3.076;P =0.000,0.037).(4) The basic serum ACTH concentrations had no significant difference between the severe group and the mild group[≤72 h: (101.55±61.52) ng/Lvs (85.54±59.83) ng/L,7 d:(54.91±22.75) ng/Lvs (71.07±20.51) ng/L,14 d: (44.67 ± 28.30) ng/L vs (44.92 ± 24.68) ng/L, all P ＞ 0.05].(5) The serum cortisol and ACTH concentration within 72 hours after birth of the dead infants[(351.9 ± 179.7) μg/L, (215.5 ± 165.9) ng/L] were significantly higher than those of the survivors [(201.4 ± 161.4) μg/L, (83.5 ± 54.0) ng/L], and there were significant differences(t =-2.547,-3.833;P =0.012,0.000).(6) The basic serum cortisol concentration within 72 hours after birth had a negative correlation with NCIS(r =-0.293, P =0.043), pH (r =-0.336, P =0.000) and base excess (BE) (r =-0.261 ,P =0.002);but it had a positive correlation with ACTH concentration within 72 hours after birth (r =0.443 ,P =0.000).The serum basic ACTH within 72 hours after birth had a negative correlation with BE (r =-0.181 ,P =0.031) ,and had positive correlation with the basic serum cortisol concentration within 72 hours and day 7 (r =0.443,0.268;P =0.000,0.048).Conclusions The term infants have the ability to response to external stimuli by regulating cortisol secretion.The basic serum cortisol concentration of term infants is related to the critical illness.The worse the condition is,the higher the base serum cortisol concentration is.