We report 16 cases of a distinctive variant of primary thymic epithelial neoplasm characterized by prominent spindling of the tumor cells. The patients were seven women and nine men aged 23 to 82 years (mean, 54 years). The lesions presented as anterior mediastinal masses without clinical or radiographic evidence of tumor elsewhere. Most patients had chest pain, dyspnea, and cough; in five patients, the tumors were asymptomatic and were discovered on routine clinical examination. Grossly, the lesions were firm, well-circumscribed, and locally infiltrative, and had a firm cut surface with foci of hemorrhage, necrosis, and cystic changes. Most of the tumors were treated by complete surgical excision. Histologically, they were characterized by a spindle cell proliferation showing varying degrees of atypia and mitotic activity. In 12 cases, transitions could be seen with areas that showed the features of conventional spindle cell thymoma. In two cases, areas showing features of poorly differentiated (lymphoepitheliomalike) carcinoma and anaplastic carcinoma could also be observed. Immunohistochemical studies in 10 cases showed strong positivity of the spindle tumor cells for CAM5.2 cytokeratin, and negative staining for a panel of antibodies including epithelial membrane antigen, carcinoembryonic antigen, actin, desmin, vimentin, S-100 protein, HMB45, CD34, CD5, and CD99. Clinical follow-up of eight patients showed an aggressive biologic behavior with recurrence, metastasis, and death by tumor in five of them 2 to 5 years after diagnosis. Based on these findings, the present tumors are interpreted as an unusual spindle cell variant of thymic carcinoma. The close association of these cases with areas showing the features of spindle cell thymoma within the same tumor mass suggests that some of these lesions may arise as a result of malignant transformation in a preexisting spindle cell thymoma.