A 31-year-old female with lymphoma was incidentally found to have a left retroperitoneal lymphangiomyomatosis (LAM). The tumor was proved by pathology and immunohistochemical study of the tissue specimen obtained by ultrasound-guided core needle biopsy. The characteristic sonographic, computed tomographic, magnetic resonance imaging, and positron emission tomographic (PET) features of this unusual lesion were described. It was managed conservatively and remained stable on 2-year follow-up study. LAM should be considered in the differential diagnoses in cases of a retroperitoneal solid mass with cystic components.