The plasma levels of 8 steroid hormones were studied longitudinally in 20 new-borns affected with either 21-hydroxylase (21-OH), 11-hydroxylase (11-OH) or 3β-hydroxysteroid deshydrogenase (3β-ol) deficiencies during the first month of life. Comparison was also made between the patterns observed according to age at first examination (n = 13 before 8 days of age) and the type of the enzymatic block. The most striking findings were the variability in hormone levels and/or evolution with age and the difficulties in making a definite positive or etiological diagnosis at first examination. In some newborns with untreated 21 -OH deficiency, 17α-hydroxyprogesterone levels may start off within the normal range or not so far above it. Also, levels of unconjugated dehydroepiandrosterone and other Δ5 steroids may be so high during the first week of life in all 3 forms that an erroneous diagnosis of 3β-ol deficiency could easily be considered. In several cases the etiological diagnosis was only ascertained by multiple steroid determination and/or dynamic tests. These discrepancies were not found in infants studied later on in life. It thus appears that a peculiar steroid pattern is observed in the immediate postnatal period in babies with various enzyme defects, which might be related to the morphological changes occurring in the adrenal cortex at this age.