Reflux of embolic material during therapeutic transcatheter embolization is a potential complication which can result in occlusion of distal vascular beds. The conditions under which reflux was demonstrated in laboratory animals include (1) low flow states, (2) overvigorous flushing, (3) selective contrast injections, and (4) placement of embolic material too proximally. Balloon occlusion of the orifice of the vessel undergoing embolization protects against reflux and allows more homogenous embolization. Because of experience gained in the laboratory, therapeutic transcatheter embolization is now performed in patients with balloon catheter protection. Preliminary clinical experience is described.