• We hypothesized that administration of the iron chelator deferoxamine would inhibit iron-catalyzed free radical generation and lessen the severity of oxygeninduced pulmonary injury. To evaluate its efficacy and safety in premature infants, we administered deferoxamine by intravenous infusion to five premature baboons with hyaline membrane disease supported with conventional ventilation and 100% oxygen for 6 days. Seven animals served as controls. Deferoxamine treatment was initiated at 10 mg/kg per hour but, after the precipitous death of the first animal, was progressively reduced to 1.25 mg/kg per hour in the other animals. Four of five deferoxamine-treated baboons developed cardiovascular collapse and all five died by 42 hours. Five of the seven control animals survived the 6-day expermental period. Since cardiovascular toxic effects have not previously been reported, these findings suggest unique vulnerability of the immature cardiovascular system to iron chelation.