We report a case of spontaneous regression of a cystic adventicial disease of a popliteal artery in a 37 years old man. The peroperative resection of the adventicia give us the histologic confirmation of the diagnostic. The cyst's content may have gone through either in the peri-arterial space or through a communication with the articulation of the knee. For us, it seems to confirm the embryologic theory for the pathogenic of this disease.