Thirty-four spontaneously breathing newborns with respiratory distress syndrome (RDS) requiring nasal continuous positive airway pressure (CPAP) and an arterial-to-alveolar oxygen tension ratio (a/A PO2) of 0.15-0.22 were randomized to treatment with nebulized surfactant (Curosurf) or to serve as controls. All children were first supported by nasal CPAP according to normal clinical routines. Surfactant was administered using a modified Aiolos nebulizer, and a total of 480 mg was aerosolized in each case. The control group received no nebulized material, but had the same CPAP support. Acid-base status and a/A PO2 were determined at regular intervals before, during and after surfactant administration. Both groups included in the study were similar with regard to gestational age, birthweight, steroids given before birth, sex and Apgar scores as well as a/A PO2 when entering the study. There were no significant differences between the groups in a/A PO2 1-12 h after randomization, number of infants needing mechanical ventilation, time on ventilator or CPAP. Two children in the treated group developed bronchopulmonary dysplasia. No side effects of the surfactant therapy were noted. No beneficial effects of aerosolized surfactant were demonstrated in our trial, contrary to data from animal experiments. This finding probably reflects differences in administration techniques. Our findings do not justify large clinical trials with the same protocol. Further work is needed to optimize delivery of aerosolized surfactant to the neonatal lung in clinical practice.