Measurements of particle number size distribution in the range 11-452 nm have been made on the side of the busy Marylebone Road in central London over a period from April 1998 to August 2001. The data have been analysed to demonstrate the influences of meteorological factors upon different size fractions and upon the overall size distribution. The relationship to traffic volumes indicates that the accumulation mode particles are associated with emissions from heavy-duty traffic (mainly diesel vehicles) whilst particles in the range 30-60 nm show a stronger association with light-duty traffic. Both of these size fractions show the anticipated dilution effect with increasing wind speed. Particles in the 11-30 nm range behave anomalously showing no clear relationships to traffic volumes and a lesser effect of dilution by increasing wind speed than for the larger particles. Particles in this fraction tend to peak in the early morning showing an inverse association with air temperature. It is concluded that this size range contains freshly nucleated particles formed as the exhaust gases are diluted with ambient air. (C) 2003 Elsevier Ltd. All rights reserved.