Twenty cases of adenomatoid spindle cell thymomas are presented. The patients are 13 males and 7 females between 7 and 82 years of age (mean: 55 y). Clinically, all patients presented with symptoms of chest pain and shortness of breath. Radiologically, an anterior mediastinal mass was discovered, and complete surgical resection was performed in all of the patients. Grossly, the tumors were described as well-defined solid tumor masses surrounded by membranous tissue, which at cut surface show a light tan homogenous surface. Areas of necrosis, hemorrhage, and/or cystic degeneration were not observed in any of the cases. Histologically, all tumors showed similar histological features and were characterized by the presence of a spindle cellular proliferation with an "adenomatoid-like" appearance, which at higher magnification showed the presence of cells with a signet-ring cell-like appearance. Rare mitotic figures were seen in some cases. Seven tumors showed transcapsular invasion, whereas 13 cases were encapsulate. Immunohistochemical studies showed positive staining for broad-spectrum keratin and keratin 7 with only scattered cells positive for calretinin and epithelial membrane antigen. Other markers including S-100 protein, desmin, smooth muscle actin, and α-feto protein were negative. Follow-up information ranging from 4 to 96 months (average: 32.3 mo) was obtained in 17 patients showing that all patients were alive. The cases herein described highlight the importance of recognizing this unusual pattern of spindle cell thymomas to avoid misdiagnosis with other tumors, namely, when dealing with small mediastinoscopic biopsies.