Reference plasma adrenal steroid levels during early infancy are frequently used to verify hormone measurements when any adrenal abnormality is suspected. We aim to obtain longitudinal reference plasma levels for 17-hydroxyprogesterone (17OHP), 11-desoxycortisol (11DOC), cortisol, dehydroepiandrosterone sulfate (DHEAS), testosterone, and androstenedione in healthy infants from birth to 6 months of age. In 138 term infants, 80 males and 58 females, plasma steroid levels were measured using specific RIA procedures at birth and on the 3rd, 15th, 30th, 60th, 90th, 120th, 150th, and 180th days of life. Smoothed percentiles for each variable were calculated according to the LMS method (LMS program version 1.16, Institute of Child Health, London). Except for cortisol, plasma levels of adrenal steroids decreased progressively from birth to 6 months of age. Plasma concentrations of 17OHP, 11DOC, and cortisol did not show gender differences, but testosterone and androstenedione were significantly higher in boys, and DHEAS levels were higher in girls. Longitudinal reference plasma levels for 17OHP, 11DOC, cortisol, DHEAS, testosterone, and androstenedione have been described in an adequate sample of healthy infants from birth to 6 months of age. These standards, displayed as smoothed percentiles, may be used as reference values in the management of congenital endocrine (adrenal or gonadal) abnormalities that appear in the first weeks of life.