During recent years many researchers have criticized the widely used scales on psychological job demands. For instance, they comment that in most cases different types of demand seem to be mixed in one measure. In this paper we analyse the scale on quantitative job demands in the recently developed Copenhagen Psychosocial Questionnaire (COPSOQ), with special emphasis on Differential Item Functioning (DIF). DIF refers to basic differences between groups of respondents, which may affect how they respond to questionnaire items. The data material for our study comprised a representative sample of Danish employees. The respondents were categorized into 32 specific jobs according to the International Standard Classification of Occupations (ISCO 1968). We analysed DIF with respect to the respondents' jobs with logistic regression analyses. These analyses showed that the items used in the original demand scale functioned very differently for different jobs in the population. The conclusion is that scales on quantitative demands are very sensitive to the choice of specific items. If many items on fast work pace and tempo are included in a scale, a number of blue-collar jobs will be identified as high-demand jobs. If on the other hand, many questions on long working hours and overtime are included, the use of the scale will result in an entirely different picture. This issue has so far received little attention in occupational health psychology. The results have wide theoretical and methodological implications for research on quantitative job demands.