Fifty‐two patients were randomized into two groups of 26 to the use of either 5 or 7/8F catheters for their first left heart cardiac catheterization. Clinical characteristics for the two groups were similar. 5F catheters were significantly inferior to 7/8F catheters in terms of torque control (P<.001), ease of engaging coronary ostia (P<.001), and quality of angiograms (P<.05). Nine patients in the 5F group required a change to 7/8F catheters for completion of the procedure. There was no difference in procedure time or fluoroscopy time between the groups. Time to haemostasis was significantly shorter in the 5F group (P<.01), but there was no difference between groups with respect to haematoma formation or rebleed after haemostasis. We conclude the slight advantage of 5F catheters in terms of haemostasis is outweighed by many disadvantages. Their routine use in cardiac catheterization, at least at this time, cannot be recommended. Copyright © 1990 John Wiley & Sons, Ltd.
