The phenomenon of reverse mismatched ventilation-perfusion on pulmonary scintigraphy is a fairly common occurrence. We present a patient who was experiencing decreasing oxygen saturation and had a reverse mismatched ventilation-perfusion imaging pattern associated with radiotracer retention in a main bronchus. Technetium-99m-DTPA aerosol lung imaging showed tracer retention in the trachea and right main bronchus, absent ventilation in the right lung, and normal ventilation in the left lung. Technetium-99m-MAA perfusion lung images showed normal perfusion of the left lung and some perfusion in the right lung. These findings represented a reverse ventilation-perfusion mismatch. Reverse mismatched ventilation-perfusion, or totally absent ventilation with preservation of some perfusion in the right lung, resulted in functional intrapulmonary shunting, which explained the decreasing oxygen saturation observed in this patient. A concurrent portable chest radiograph showed elevation of the right hemidiaphragm, a shift of the mediastinum to the right, deviation of the endotracheal tube, narrowing of the intercostal space of the right thorax, and collapse of the right lower lobe. The radiographic findings of underventilation of the right lung with atelectasis of the right lower lobe were due to mucous plugging the right main bronchus.