Thromboembolic complications occurred during diagnostic coronary arteriography in three patients in spite of systemic heparinization. These mishaps were associated with the use of a nonionic contrast medium, which is known not to have a significant protective effect on coagulation pathways. If nonionic contrast media are to be used in angiographic studies of critical organs, meticulous technique is imperative, and adequate systemic heparinization is recommended.