Cerebral embolization is a serious complication during diagnostic heart catheterization. To date there have been no studies to determine whether the technique and the catheter type influence the frequency of cerebral microembolic signals (MES's) during left ventricular catheterization. Twenty-two patients had a leading straight tip guidewire protruding 5 to 10 cm outside the coronary catheters when the latter was advanced over the aortic arch (group A), whereas in 21 patients the guidewire was withdrawn in the descending part of the aorta (group B). Transcranial Doppler of the left middle cerebral artery was performed to monitor the number of cerebral MES's. When a protruding guidewire was used to advance the coronary catheters over the aortic arch, MES's were detected in 86% of the patients compared with 29% when the catheters were advanced without a guidewire (relative risk = 4.6, p = 0.00001). The number of MES's per patient also was significantly higher when a guidewire was used (median 9 vs 0) (p = 0.000004). In group A, a higher number of MES's was detected when a right Judkins catheter was advanced over the aortic arch than when a left Judkins catheter was advanced (median 6.5 vs 1) (p = 0.0005) and in patients who previously had a myocardial infarction than in those who had not (median 1 1 vs 4) (p = 0.007). This study strongly suggests that the risk of embolization is greater when straight tip guidewires are used to advance catheters over the aortic arch during left ventricular heart catheterization, especially in patients with a history of myocardial infarction.