Thymolipoma is a benign and rare tumor that could be found at any age. Thymolipoma associated with the myasthenia gravis, Graves disease, aplastic anemia, and hypogammaglobulinemia was reported previously, but in this case, thymolipoma is associated with lymphocytosis. A 6-year-old girl was brought to the hospital because of a chronic cough. Her evaluation revealed a 130 × 160× 160 mm fat-containing soft tissue mass arising from anterior mediastinum with complete left lung collapse and contralateral mediastinal shift. Her past medical history showed that she had been evaluated and treated unsuccessfully due to severe lymphocytosis two years earlier. Her peripheral blood and bone marrow cell morphology were normal; in contrast, blood cell count and CD flow cytometry showed severe lymphocytosis. The patient's tumor was excised entirely without any complications, and lymphocytosis resolved during the follow-up period. Because the T lymphocytes are developed in the thymus, and more than 80% of cells in CD flow cytometry were T lymphocytes, and the lymphocytosis resolved with tumor removal; therefore, the authors suggested that Thymolipoma could be associated with lymphocytosis.