Abstract: Objective　To explore adrenal insufficiency in the preterm infants who had critical lung diseases and the relationship between adrenal insufficiency and long-term or short-term prognosis. Methods  The basal cortisol and ACTH of fifty preterm infants enrolled were tested. The peak cortisol at 30 minutes after low dose ACTH stimulation was tested. The adrenal insufficiency was defined as the concentration of the basal cortisol less than 15 μg/dl or the increment of cortisol concentration after low dose ACTH stimulation less than 9 μg/dl. The pretern infants with adrenal insufficiency were the AI group and others were the adrenal normality, AN group. Results  Thirty-one cases were adrenal insufficiency in 50 preterm infants with critical lung disease. The incidence was 62%. The concentrations of the basal and peak cortisol in AI group were lower than those in AN group (p<0.01). The increment of cortisol concentration after low dose ACTH stimulation had not significant difference between two groups. The concentration of ACTH in AI group was lower than that in AN group (p<0.01). The mean arterial blood pressure in AI group was lower than that in AN group (p<0.01). The blood sugar, potassium and sodium had no significant difference between two groups (p>0.05). There was no significant difference in the mortality and the incidence of BPD between two groups (p>0.05). Conclusions  The incidence of adrenal insufficiency was high in the preterm infants with critical lung diseases, which may be due to insufficient production of endogenous ACTH. When the adrenal insufficiency was happened, the blood pressure is relatively low in preterm infants with critical lung diseases, but it was not related to the mortality and the incidence of BPD.