A type of malignant neoplasm of not frequent observation is reported and the difficulties concerning the distinction from the benign counterpart are underlined, for which a detailed instrumental study is made necessary, integrated with pre and, above all, postoperative histologic analysis. A white male patient of 83, affected by plurirecidivant chondromyxoma of the rib, was admitted for local recurrence. The lesion objectively appeared not dissimilar from the preceding ones, as an ovalar mass fixed on the underlying plains. Radiologic and ultrasonographic examinations seemed to confirm the admission diagnosis; at the end the patient underwent a new intervention. Histologic examination of the lesion yielded a diagnosis of chondromyxosarcoma. The clinical case is made even more interesting by the finding, through the TC technique, preoperatively performed, of a metastatic repetition on the opposite side. The authors examine the problem of the early recognition of a malignant neoplasm developing, in a patient with a clinical history of recidival chondromyxoma. Physical examination and laboratory analysis are not useful in the assessment of the occurring transformation. At molecular level chondromyxosarcoma is characterized by several genomic rearrangements and mutations. Though primitive chondromyxosarcoma often involves the ribs, chondromyxoma's transformation into its malignant counterpart is not a common event. The development of distant metastasis characterizes long term cases. Given the lack of encouraging results about chemo and radio-therapy efficacy in influencing the natural course of the disease, a systematic approach will be made necessary in patient management