The synovial lipoma are uncommon articular tumors of unknown origin. We report two cases of synovial lipoma arborescens arisen in a man and a woman respectively 41 and 27 years old. Both had already consulted for a tumefaction progressively increasing in volume. It was accompanied in the first case by a laxity in the knee joint. Radiography and arthroscopy showed an important hyperplasia of the articular synovia. A total synovectomy was performed in both cases. The synovia had a hairy aspect and was extremely thickened. It weighted 1.5 kg in the first case and its section had a fatty aspect. The histological examination confirmed the diagnosis of synovial lipoma arborescens. The observed aspect of the two tumors and particularly their volume, which was very important in the first case are arguments in favor of their tumoral nature.