Preliminary experience with 5 and 6 French diagnostic catheters as guiding catheters for coronary angioplasty. With the reduction in profile of balloon dilation catheters, until recently, it has been the internal dimensions and performance of the guiding catheter that has mandated the use of 7, 8 or 9 French (F) systems for the performance of percutaneous transluminal coronary angioplasty (PTCA). A new 5F catheter design (Sherwood Medical Co., St. Louis, MO) provided a large inner lumen (0.4") permitting use of 0.20-0.22" fixed-wire PTCA balloon catheters with good coronary visualization. Potential advantages include reduced coronary artery ostial trauma and catheter induced damping and enhanced patient comfort. We report our initial experience in 14 patients undergoing PTCA with a 5 and 6F guide/fixed-wire system. Mean age was 63 +/- 10 (43-78 years). PTCA indications: Cardiogenic shock (1), post-myocardial infarction angina pectoris (2), grade III angina (5) and unstable angina pectoris (6). Vessel attempted: Left anterior descending (3), circumflex (4), obtuse marginal (2), diagonal (1), right coronary artery (3), and internal thoracic artery (1). Twelve patients had femoral approach; two brachial approach. The USCI Probe (USCI Division, Billerica, MA) was used in 8 lesions and SCIMED ACE (SCIMED Life Systems, Maplegrove, MN) catheter in 7 lesions. Successful 5 or 6F guide/fixed-wire dilations reduced the stenosis (77 +/- 14 to 37 +/- 30%) and were successfully performed in 79% (11/14). One 5F patient required 8F guiding catheter and was dilated with 2.0 fixed-wire balloon. A second failed 5F PTCA could not be dilated with any larger conventional system. A third total occlusion could not be crossed with a guidewire or fixed wire balloon. No patient had a complication. 