A case is presented of lipoma arborescens of the knee with an atypical, sudden onset. Arthrography and ultrasound confirmed a synovially based lesion in the suprapatellar pouch. CT enabled a preoperative diagnosis of lipoma arborescens to be made, which was confirmed by surgical excision of a fronded fatty mass. Histological studies showed hyperplastic villi with mature adipose cells in the subsynovial layer. Resection of the synovium is considered curative, with only one case of recurrence after synovectomy having been reported [4].