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A case of a ventilation-perfusion mismatch seen postoperatively in a patient with chest pain is reported. There was absence of perfusion to the right lung, with relatively normal ventilation. The study was initially interpreted as indicating a high probability of pulmonary embolus. It was then discovered that the injection had been inadvertently made into a Swan-Ganz catheter, with its tip in the left main pulmonary artery. The mismatch was therefore iatrogenic and not related to pulmonary embolus. The false-positive lung imaging which resulted has not been previously reported in the literature.