We present a case of de novo fibrosarcoma in a 43-year-old male, with MRI documented evolution from a 5 mm hyperintense area to 5 cm tumor mass in a 12-month period. The diagnosis of low-grade fibrosarcoma was established by three experienced neuropathologists. The patient underwent gross total resection with adjuvant fractionated conformal radiotherapy. Following first recurrence 3 months later, the patient was reoperated and stereotactic radiosurgery of a residual tumor was performed thereafter. The pathological diagnosis was similar, but with additional extensive radiation effects. Six months later the patient underwent aggressive surgical resection for second recurrence. The pathological diagnosis was WHO grade IV glioblastoma. The etiology of this highly unusual progression from primary mesenchymal neoplasm to high-grade glioma is discussed.