In a clinical, uncontrolled study, bovine surfactant was administered intratracheally to 32 very low birth weight infants. In the first 18 patients, the dose was 20-40 mg/kg body weight (group 1, median birth weight 750 g) in the other 14 infants 40-50 mg/kg (group 2, median birth weight 840 g). The bovine surfactant was given, if the peak inspiratory pressures (PIP) were above 22-27 cm H2O depending on the infant's birth weight or whether the fraction of inspired oxygen (FiO2) was greater than 0.5. The FiO2 decreased from the pretreatment value of 0.7 to 0.46 after 1 hour, whereas PIP could not be lowered as rapidly as FiO2 (PIP from 29 to 26 cm H2O after 1 h). Surfactant treatment was more effective in group 2 comparing the reduction in FiO2 (delta FiO2 0.34 versus 0.16 in group 1 after 1 h), survival in group 2 was higher (71%) than in group 1 (56%). Our data are consistent with those of other groups using other natural surfactant preparations.