Objectives: To investigate the effect of hydrocortisone treatment on survival without bronchopulmonary dysplasia (BPD) and to study whether serum cortisol con centrations predict the response. Study design: We performed a randomized, place bo-controlled trial on infants with gestation ≤30 weeks, body weight of 501 to 1250 g, and respiratory failure. Hydrocortisone was started before 36 hours of age and given for 10 days at doses from 2.0 to 0.75 mg/kg per day. Shortly befor e hydrocortisone treatment, basal and stimulated (ACTH, 0.1 μg/kg) serum cortis ols were measured. Results: The study was discontinued early, because of gastroi ntestinal perforations in the hydrocortisone group (4/25 vs 0/26, P =. 05); 3 of the 4 had received indomethacin/ibuprofen. The incidence of BPD (28%vs placebo 42%, P = 0.28) tended to be lower, and patent ductus arteriosus (36%vs 73%, P =. 01)was lower in the hydrocortisone group. The hydrocortisone-treated infan ts with serum cortisol concentrations above the median had a high risk of gastro intestinal perforation. In infants with cortisol values below the median, hydroc ortisone treatment increased survival without BPD. Conclusions: Serum cortisol c oncentrations measured shortly after birth may identify those very high-risk in fants who may benefit from hydrocortisone supplementation. Objectives: To investigate the effect of hydrocortisone treatment on survival without bronchopulmonary dysplasia (BPD) and to study whether serum cortisol con centrations predict the response. Study design: We performed a randomized, place bo-controlled trial on infants with gestation ≤ 30 weeks, Hydrocortisone was started before 36 hours of age and given for 10 days at doses from 2.0 to 0.75 mg / kg per day. Shortly befor e hydrocortisone treatment, basal and stimulated (ACTH, 0.1 Results: The study was discontinued early, because of gastroi ntestinal perforations in the hydrocortisone group (4/25 vs 0/26, P =. 05); 3 of the 4 had received indomethacin / ibuprofen. The incidence of BPD (28% vs. placebo 42%, P = 0.28) tended to be lower, and patent ductus arteriosus (36% vs 73%, P = .01) was lower in the hydrocortisone group. infan ts with serum cortisol concentrations abo ve the median had a high risk of gastrointestinal perforation. In infants with cortisol values ​​below the median, hydrocortisone treatment increased survival without BPD. Conclusions: Serum cortisol c oncentrations measured shortly after birth may identify those very high-risk in fants who may benefit from hydrocortisone supplementation.