Coronary balloon angioplasty through diagnostic 6 French catheters. We investigated the use of ultralow profile balloon catheters (Scimed ACE, USCI Probe, Cordis, Orion) for coronary angioplasty through 6 French diagnostic catheters (Schneider, Cordis). Contrast injection was assisted with a Hercules pump (Cordis) in all cases. During 21 procedures, angioplasty of 27 lesions in 20 selected patients was attempted (1.3 lesion/procedure). Twelve lesions were in the right, 10 in the left anterior descending, and 5 in the left circumflex coronary artery. Balloon size varied between 2.5 and 3.5 mm. Twenty lesions could be successfully dilated (74%) through the 6 French catheter and 7 lesions required an exchange to a 7 French angioplasty guiding catheter. For 5 cases, another balloon was also necessary to complete the procedure. The final overall success rate was 100% per patient and per lesion and there were no major complications. Despite the small internal catheter lumen (1.22 mm) coronary visualization was adequate, and mechanical support was good. Failures of 6 French catheters were attributed to insufficient torque control and excessive friction when the balloon crossed the tapered end of the diagnostic catheter. Coronary angioplasty through a diagnostic 6 French catheter is feasible and may represent a reasonable alternative for simple cases that are done during the same session as the diagnostic angiography. Once available, 6 French high flow angioplasty guiding catheters without a tapered tip should improve success while retaining the advantage of a small femoral puncture site. 