Background and objectives: Serum 17 alpha- hydroxyprogesterone (17OHP) and bilateral adrenal sizes are pivotal for clinical practice in both diagnosis and treatment of congenital adrenal disorders during the first month of life. Our aims were to determine the reference ranges for serum 17OHP and bilateral adrenal gland sizes according to sex and age groups in healthy term newborns. Materials and methods: A total of 156 healthy newborns, aged 4-7 days (Group 1) or 26-30 days old (Group 2) were included in the study. Serum 17OHP concentration was measured in the morning by radioimmunoassay. The right and left adrenal glands' width, length, and depth were measured with ultrasonography by the same radiologist and the volumes were calculated. Results: The clinical characteristics and serum 17OHP concentrations were similar in male and female newborns. Percentiles for serum 17OHP concentration and the volume of adrenal glands according to age groups and sexes were obtained. Mean 17OHP concentration was 4.67 +/- 2.6 ng/ml and 4.49 +/- 2.7 ng/ml at the first and fourth week of life, respectively (p > .05). There was a significant decrease in adrenal sizes during the fourth week of life. There was no significant correlation between serum 17OHP concentration and adrenal gland sizes. Conclusions: We have determined reference intervals for serum 17OHP concentration and bilateral adrenal gland sizes for healthy newborns. Although serum concentrations of 17OHP did not change significantly through the first month of life, our reference intervals for serum 17OHP concentration and adrenal sizes may improve clinical approach toward newborns who are suspected of adrenal disorder. We conclude that our reference intervals can guide for congenital adrenal screening regarding serum 17OHP concentration besides diagnosis of adrenal hypoplasia or hyperplasia with ultrasonographic adrenal gland sizes.