Here, three cases of combined lipoma arborescens (LA)--a rare neoplastic lesion of the synovium--and osteoarthritis (OA) with bilateral knee involvement in elderly patients are reported. The clinical, magnetic resonance imaging and histological characteristics of this rare condition are described. Limited by the similarity of symptoms between LA and OA and the low sensitivity of X-ray for discerning LA, it was found that a diagnosis of LA could be easily missed. Follow-up data suggested that the treatment of LA was not jeopardized even if the diagnosis was not made preoperatively, since the lesion is easily discerned intraoperatively. The definitive treatment of synovectomy can then be performed without recurrence of the lesion. Based on the evidence of the sequence of symptoms and the progression of untreated LA with static OA at follow up, it is suggested that the aetiology of LA is secondary to OA in elderly patients.