Leiomyosarcoma of the renal vein is a rare and malignant tumor difficult to diagnose. No standard treatment has been defined and prognosis is poor. We described the case of a 68-year-old woman with rheumatoid arthritis treated by methotrexate who developed a leiomyosarcoma of the left renal vein with a fatal outcome in less than 1 year. Association of a leiomyosarcoma and rheumatoid arthritis raises the question of a casual association or of a predisposing factor since studies have shown increased risk of cancer with this rheumatism.