We performed a randomized, prospective clinical trial comparing intratracheal administration of human surfactant with conventional treatment with intermittent mandatory mechanical ventilation alone for treatment of severe respiratory distress syndrome in preterm infants of less than 30 weeks gestation. Twenty-two infants (mean gestational age 27.0 weeks, mean birth weight 987 gm) were given surfactant, and 23 infants (mean gestational age 27.2 week, mean birth weight 1055 gm) received intermittent mandatory ventilation. Infants given surfactant required less FiO2 during the first week, had lower mean airway pressure during the first 48 hours, and had improved ventilatory index and a/A PO2 ratio. Death or the occurrence of bronchopulmonary dysplasia was significantly less among infants given surfactant (P = 0.019). Pneumothorax, pulmonary interstitial emphysema, and need for FiO2 greater than or equal to 0.3 for greater than 30 days was significantly less in the surfactant group. This trial confirms the efficacy of treatment with human surfactant in preterm infants with severe respiratory distress syndrome.