A 29-year-old woman was admitted to our hospital because of hemoptysis and dyspnea. A chest radiographic study revealed an enlargement of the right lung hilum, and the left pulmonary artery could not be clearly visualized on contrast-enhanced CT scan. Both perfusion scintigraphy and pulmonary angiography confirmed the absence of the left pulmonary artery, and agenesis of the right upper pulmonary artery. Her clinical status has been stable for the previous two years of follow-up, even though she required long-term oxygen therapy. This is a rare case of a young woman with the unilateral absence of the left pulmonary artery incidentally found after an episode of hemoptysis.