The effect of experimental balloon dilation on seven excised specimens of classic coarctation was assessed histologically. Widening of the lumen was achieved in all specimens by tears in the vessel wall. The tears extended through the intima in one specimen and through both intimal and medial layers in six specimens. In view of the extent of wall damage, we postulate that there would be serious long-term complications following balloon dilation for the treatment of native coarctation.