To determine the effect of prolonged dexamethasone therapy on oxygen-dependency clinical phase of prematures with bronchopulmonary dysplasia, we examined a consecutive group of 27 infants (birth weight, < 1500 g and gestational age, < 32 weeks), who remained with a static or deteriorating oxygen-dependency after weaning from the respirator [pre-treatment FiO2, mean +/- SEM over three days (31 +/- 2)%, range (27-73)%]. Twenty five out of 27 infants were weaned from supplemental oxygen during the 42-day steroid treatment period, with a mean (+/- SEM) duration of oxygen supplementation of 16 +/- 4 days. The distribution of the ratios of successive post-treatment FiO2 values with respect to pre-treatment FiO2 shows, on average, a progressive reduction with time. The percentage of the FiO2 decrease is statistically significant at a level of 2 and 3 SD after 3 days and 7 days, respectively and the average FiO2 fall, as function of time, follows an exponential law. It follows that the time spent in oxygen for a single patient may be determined, accounting for the individual severity of pre-treatment FiO2.