Objectives. To investigate whether aerosolized prostacyclin improves oxygenation in children with acute lung injury. Design. Double-blind, randomized, and placebo-controlled trial. Setting: Pediatric intensive care unit at a university hospital. Patients: Fourteen children with acute lung injury defined by the criteria of an American-European Consensus Conference. Interventions. Aerosolized prostacyclin (epoprostenol sodium) by stepwise increments of different doses (10, 20, 30, 40, and 50 ng(.)kg(-1.)min(-1)) vs. aerosolized normal saline (placebo). Measurements and Main Results: Before the start of the study, and before and after each dose of prostacyclin/placebo, the following variables were measured: arterial blood gases, heart rate, mean arterial blood pressure, and ventilator settings required. Changes in oxygenation were measured by calculation of the oxygenation index (mean airway pressure x 100 x Pao(2)/FIO2). After treatment with aerosolized prostacyclin, there was a significant 26% (interquartile range, 3%, 35%) improvement in oxygenation index at 30 ng(.)kg(-1.)min(-1) compared with placebo (p = .001). The response to prostacyclin was not the same in all children. We saw an improvement of greater than or equal to20% in eight of 14 children (i.e., responders), and the number needed to treat was 1.8 (95% confidence interval, 1.2-3.2). No adverse effects were observed. Conclusions. Aerosolized prostacyclin improves oxygenation in children with acute lung injury. Future trials should investigate whether this treatment will positively affect outcome.