A regional key comparison for the calibration of cylindrical diameter standards was carried out within EUROMET between 1996 and 1998. Twelve laboratories from European national metrology institutes took part, the Swiss Federal Office of Metrology (OFMET) acting as the pilot laboratory. The standards to be calibrated were three ring gauges of diameters 3 mm, 40 mm and 90 mm, and two plug gauges of diameters 2.5 mm and 50 mm. The measurand to be determined was the diameter of the standard in a given direction at three heights. Owing to the large variety of measuring instruments used by the participating laboratories, ranging from highly specialized, state-of-the-art equipment to commercial, multi-purpose length-measuring machines, the stated uncertainties vary to a large extent. Since the corresponding key comparison (CCL-K4) of the Comité International des Poids et Mesures (CIPM) has not yet started, it was appropriate to calculate preliminary reference values for this regional comparison. The deviations with respect to these preliminary reference values are in most cases smaller than the expanded uncertainty of these differences.