In quantitative coronary arteriography (QCA), the outer diameter of the contrast catheter is used almost exclusively as the calibration device. Since the true outer diameter of these catheters are smaller than the French sizes listed by the manufacturer, it has been advocated to measure the actual sizes with a precision micrometer. However, this represents a significant effort in the organizations of the QCA Angiographic Core Laboratories and of the participating cardiac catheterization laboratories. In the Dutch REGRESS trial on regression/progression of coronary artery disease, we have measured a total of 2,104 catheters from four different manufacturers. In 2,048 of the 2,104 catheters the variabilities in the measurements were 0.02-0.03 mm, which represents an error of only 1% of the average catheter sizes. In the remaining 56 catheter measurements, the variability was 0.04 mm. From these data we may conclude that from now on the stated mean values can be used for calibration purposes. This has important practical consequences for running clinical trials and QCA Core Laboratory operations.