Total cortisol as well as percentage and absolute free cortisol values were determined in 75 full-term, 88 premature and 38 small-for-age (born at term) infants in the first 3 months of life. Equilibrium dialysis and radioimmunoassay were used to estimate the percentage value of the unbound fraction and the value of total cortisol from which absolute free cortisol level was calculated, respectively. A systematic decrease in the free cortisol value was observed in all the three groups of infants during the study period (in full-term infants from 32.3 to 19%, in prematures from 36.6 to 20.8%, and in small-for-age infants from 32.3 to 19.2%). A comparison between the percentage values of free cortisol in the groups studied revealed only slight differences which were not significant. The absolute free cortisol values in full-term infants were highest immediately after birth (4.05 μg/dl), then they fell to the lowest level of 0.67 μg/dl observed between the third and fifth days of life, and increased afterwards reaching the level of 1.89 μg/dl in the third month. The absolute free cortisol values in premature newborns at 3–5 days of life exceeded the values observed in full-term subjects. The pattern of free cortisol in the prematures seems to be ‘delayed’ as compared with that in full-term newborns. The absolute free cortisol values in small-for-age infants were much more similar to those found in the full-term subjects, than to those in premature babies.