Experimental vehicle idle emission factors have been compared with calculated factors for 100 vehicles in Madrid. The vehicles were selected according to the distribution (by age and fuel type) of the official, national car fleet (up to 31 December 1997). They were then clustered following the criteria of the Computer Programme to Calculate Emissions from Road Transport (COPERT III) category classification. NO(x) and CO idle emission factors show a wide range of values in each category and an important deviation between the experimental values and the corresponding parameterised ones. Fifteen percent of tested vehicles in this study have been identified as gross emitters generating approximately 50% of the total CO emission volume.