Studies mainly from other countries have demonstrated that traffic participation may lead to high levels of exposure to air pollution. These studies have so far paid little attention to ultrafine particles, mainly due to the complexity of measurements. This is a serious limitation, as concentrations of ultrafine particles are much more elevated near traffic compared to concentrations of fine particles and gases such as nitrogen dioxide. Moreover toxicological and epidemiological studies have provided indications that exposure to ultrafine particles may lead to effects on illness/mortality as a result of coronary and vascular disease and respiratory disease. In addition, various studies have related ‘living near busy roads’ to effects on respiratory diseases and premature mortality. High exposure to ultrafine particles may provide an explanation for these relations. This report describes ultrafine particle measurements during cycling and driving: a description of the height of the concentrations and an analysis of the factors leading to high concentrations.