Soft tissue sarcomas account for 1-2% of all tumours. About 10 to 20% of these tumours are localized in the retroperitoneal space. The basis of diagnostics is radiological examinations. The treatment of retroperitoneal sarcomas is based on surgery combined with radiotherapy and chemotherapy. The patient, aged 45, with a histopathological diagnosis of liposarcoma, was operated on with brachytherapy and radiotherapy. After 18 months the recurrence of the disease was diagnosed in CT examination. Before chemotherapy, CT and MR examinations and biopsy had been done with no recurrence result. The above-mentioned changes were found to be the post-treatment changes. The current condition of the patient is good; there is no recurrence and no remote metastasis.