We encountered a 69-year-old woman displaying a filling defect within the left descending pulmonary artery (PA) on a chest CT scan and pulmonary angiography. A subsequent 2-[18F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) scan demonstrated focal uptake in the left hilum. A cytologic examination of transbronchial needle aspiration specimens revealed small cell carcinoma. The patient underwent concurrent radiation therapy and chemotherapy with cisplatin and etoposide, resulting in tumor shrinkage and recanalization of the involved PA. This is the first case of small cell carcinoma localized exclusively within the PA, and positive findings on FDG-PET facilitated the unexpected diagnosis.