Nondiagnosed left main ostial stenosis partly due to the use of 5 French coronary angiographic catheters. Two cases have been reported in which the use of 5 F angiographic catheters is associated with a failure to diagnose an ostial stenosis of the left main coronary artery (LMCA). In both cases, the erroneous diagnosis led to an inappropriate indication for percutaneous transluminal coronary angioplasty (PTCA) on other stenosed vessels, and the ostial left main lesion was unexpectedly discovered when using 8F guiding catheters. It is supposed that the ability of performed 5F catheters to pass easily through an ostial lesion makes detection of such proximal stenosis much more difficult. We suggest that the choice of 5F catheters must be approached with caution when left main disease is potentially expected from the clinical features. 