The aim of this study is to contribute to the emerging field of quantification of Health Impact Assessment (HIA), by analysing how different relative risks affect the burden of disease for various socio-economic groups (SES). Risk analysis, utilising attributable and impact fraction, raises several methodological considerations. The present study illustrates this by measuring the impact of changed distribution levels of smoking on lung cancer, ischemic heart disease (IHD), chronic obstructive lung disorder (COLD) and stroke for the highest and lowest socio-economic groups measured in disability adjusted life years (DALY). The material is based on relative risks obtained from various international studies, smoking prevalence (SP) data and the number of DALY based on data available for Sweden. The results show that if smoking would have been eliminated (attributable fraction, AF), the inequality between the highest and lowest socio-economic groups may decrease by 75% or increase by 21% depending on the size of the relative risk. Assuming the same smoking prevalence for the lowest socio-economic group as for the highest (impact fraction), then the inequality may decrease by 7-26%. Consequently, the size of the relative risk used may have a significant impact, leading to substantial biases and therefore should be taken into serious consideration in HIA.