A complication of transseptal left atrial catheterization is a transient ST-elevation in the ECG with unknown cause. In this case simultaneous angiography showed a so-called "slow flow" of dye dilution in the coronary artery. A coronary spasm or an embolic occlusion of the coronary artery could be excluded. The reason for this phenomenon might be a reflex increase of the arteriolar resistance with transient ischemia.