A case of left main occlusion due to thrombus during coronary angiography is reported. This rare and extremely dangerous complication was successfully managed with resuscitation maneuvers, hemodynamic support, urgent balloon angioplasty plus stent implantation, anticoagulation with heparin and potent therapy directed at preventing platelet aggregation. The thrombus most likely detached from the arterial sheath, although coexisting circumstances, such as the use of non-ionic contrast or a transient hypercoagulable state, may have had an influence. A careful technique during the procedure is probably the most effective preventive measure.