We describe positional changes in oxygenation in two patients with respiratory failure due to massive pulmonary embolism. In both patients, oxygenation improved when the "sick" lung was dependent and deteriorated when the "healthy" lung was in the dependent position. These positional changes are different from previously reported changes in unilateral pulmonary disease. We speculate that a combination of unilateral pulmonary embolism and mechanical ventilation was responsible for the improvement in gas exchange when the "sick" lung was placed in the dependent position. The relative contribution of these two components to the development of this phenomenon is unclear.