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Synovial lipomatosis (SL; lipoma arborescens) is a tumor-like condition with villous proliferation of the synovium. The exact etiology of SL is still unknown. The knee is the most commonly involved part. The goal of this retrospective study was to report the results of arthroscopic synovectomy in patients with SL in their knee joints as well as to emphasize the importance of considering this pathology when treating patients with recurrent knee joint effusions. In total, 21 patients (8 females and 13 males) were evaluated retrospectively from May 2009 to July 2014. The mean follow-up period was 29.13 (range, 12-61) months. The mean duration of compliance was 22.76 (range, 7-61) months. All patients were evaluated by Knee Society score pre- and postoperatively. The mean preoperative and last follow-up Knee Society scores were 67.82 and 88.23 points, respectively. There was a significant difference between the preoperative and postoperative Knee Society scores (p = 0.0001). Histopathological examinations showed that the subsynovial layer exhibited diffuse replacement by mature fat cells that had formed villous projections. In addition, infiltration of mononuclear inflammatory cells was observed at different stages. SL is a nondestructive and benign tumor pathology of the knee joint without a clearly identified etiology. The main difficulty in diagnosis is lack of clinical practice.