Cystic adventitial disease (CAD) of the popliteal artery with spontaneous regression is rare. We describe a 44-year-old man with rapid onset of severe intermittent claudication who is currently undergoing conservative follow-up. CAD was diagnosed, and resection of the lesion with autologous vein replacement was scheduled. However, the claudication suddenly improved at 5 weeks after onset. Computed tomography and ultrasound revealed that the cystic lesion in the adventitia had nearly disappeared. This case report describes the imaging findings and the possibility of conservative treatment. CAD can occur in the popliteal artery and is usually treated surgically. However, reports of spontaneous regression are rare. We report our experience with a case of CAD that eventually spontaneously regressed.