Soft tissue sarcomas are very uncommon types of tumors, with their embryological origin in the mesoderm and in nerve structures of the neuroectodermic layer. They represent only 1.5% of cases in the National Registry of Malignant Tumors in Mexico. They can be encountered anywhere connective soft tissue is found. Because of their specialized localization, retroperitoneal soft tissue sarcomas have a propensity to remain asymptomatic for long periods of time and reach a large size before being diagnosed. The only accepted treatment is wide surgical excision with clear margins, without a clear benefit for adjuvant treatment. The very uncommon nature of these tumors and their varied histopathology, site and behavior classify them as a difficult entity in terms of treatment. We present here the case of a 66-year-old female with a left-side retroperitoneal tumor, complaining only of vague abdominal pain as the presenting symptom. A CT-guided needle biopsy reported a sarcoma and the patient was subjected to laparatomy with complete resection of the tumor (30 x 13 x 10 cm). Histopathological report demonstrated a low-grade retroperitoneal sarcoma and free macroscopic and microscopic borders, without obvious invasion except for left kidney and ureter. The patient refused adjuvant treatment, and she is disease-free 7 years after treatment. Retroperitoneal sarcomas can cause pain and reach very large sizes. The best treatment available is wide surgical resection with clear margins. The most important prognostic factors are free margins, type of resection, age of patient and tumor histology.