We report a case of neoplasia of pulmonary vein in a 45-year-old woman who presented with increasing dyspnea. As a consequence, the neoplasia filled the entire left atrium and appeared to be attached to the left superior pulmonary vein on surgical excision. Histologically, it was composed of a proliferation of sarcomatous cells, with a high mitotic rate and diffuse immunohistochemical positivity for smooth muscle actin, consistent with a leiomyosarcoma. The microscopic, immunohistochemical, and ultrastructural findings are discussed.