AIM: To describe the case of a thymic carcinoma with atypical clinical behavior that has arisen with a voluminous metastasis at the right hemidiaphragm while the primitive thymic neoplasm was initially occult. CASE REPORT: A 42 years female patient came to clinical observation for a voluminous thoraco-abdominal mass located in right side, infiltrating the diaphragm. The patient was submitted to surgical excision of the mass; definitive histological examination: non-keratinizing spinocellular carcinoma suggestive for neoplasia on ectopic thymic tissue or metastasis from carcinoma of the thymus. Three 3 months after surgery MR and CT-scan restaging identified the presence of anterior mediastinal mass of about 3 cm of diameter, compatible with thymical origin; thymectomy was performed (histology: Lymphoepithelial thymoma). Eight months after the first surgical procedure a restaging by CT, MR and PET CT showed the presence of disease recurrence at the right diaphragmatic level. The patient underwent surgical exploration, with right thoracotomic approach: a metastasis in the hepatic segment VII was found and radically removed. Six months after liver metastasis resection, CT scan showed disease progression in mediastinum, with involvement of pericardium and aorta; the patient died for disease recurrence five months later, 22 months after the first surgical procedure. CONCLUSION: Thymic neoplasms are the most common tumors of the mediastinum; a small percentage of these tumors are however extremely aggressive carcinomas. Rare but not exceptional findings are also cancers arising from ectopic thymic tissue.