There are two types of risk: absolute and relative risks. An individual’s absolute risk of venous thrombosis refers to the actual likelihood of getting the disease, and is not compared to any other risk. They may be expressed as rates (incidence rate) or percentages (cumulative incidence). For example, large population studies have shown that the incidence rate of venous thrombosis is around 1·5 per 1000 person-years and that an individual’s absolute lifetime risk of venous thrombosis is approximately one in nine or 11% (Heit et al, 2001; Naess et al, 2007). As shown in Fig 1, the absolute risk of venous thrombosis varies throughout life. On average, the incidence rate in childhood is 1 per 100 000 per year which increases exponentially to nearly 1 per 100 person-years in old age (Fig 1) (Naess et al, 2007). Reports that state that a factor increases or reduces the risk of venous thrombosis refer to rate differences or rate ratios, i.e. relative risk, as a comparison between effects of different exposures. (For convenience we use the term relative risk throughout this paper, irrespective of the relative risk type, such as odds ratio, rate ratio or hazard ratio. Explanation of how these different relative risk estimates are calculated and in what way these estimates may differ from one another can be looked up in epidemiological textbooks, e.g. Rothman, 2002.) A relative risk can tell us, for example, whether women have a higher risk of venous thrombosis than men. From Fig 1 it can be calculated that the risk of venous thrombosis in women compared to men is increased 1·2-fold, or that the relative risk is 1·2. In other words, the risk of venous thrombosis in women is 20% higher than in men. Relative risks are used particularly for understanding aetiology of disease, where the size of the relative risk reflects the strength of the association. The absolute risk usually determines the clinical implications. Consider air travel and venous thrombosis. Long haul flights are associated with a threefold increased risk of symptomatic venous thrombosis in the subsequent 8 weeks (Chandra et al, 2009), which indicates a moderate to strong association. However, as the overall absolute risk is 1 in 4600 over the time frame shortly after the flight (Kuipers et al, 2007a), a clinical implication, such as the use of temporary thromboprophylaxis, is probably not justified as the number of individuals needed to treat to prevent one case of venous thrombosis is too high, relative to the risk of serious side effects, such as major bleeding, induced by anticoagulants (Rosendaal, 2006). When we consider only severe pulmonary embolism directly after air travel this example becomes even more extreme: the relative risk of severe pulmonary embolism in passengers who flew more than 10 000 km compared to travellers who flew <5000 km was very high in a French study: a more than 400-fold increase (Lapostolle et al, 2001). However, the absolute risk of severe pulmonary embolism immediately after air travel was only 4·8 per million passenger arrivals in flights longer than 10 000 km (Lapostolle et al, 2001). Extrapolating these results to the Dutch population, we calculated that only one Dutch person per year travelling further than 10 000 km is seen with severe pulmonary embolism at an emergency department after disembarkment (Lijfering & van der Meer, 2009). Thus, although air travel and severe pulmonary embolism are probably causally related (as demonstrated by the high relative risk), the clinical implication of this finding seems limited due to the low absolute risk. For certain a-priori high risk groups, such as patients with previous venous thrombosis, this may be different. If such individuals have a high absolute risk after long haul flights compared to the overall absolute risk of venous thrombosis in long haul air travellers, they may benefit from temporary thromboprophylaxis. Information on absolute risk estimates for venous thrombosis after air travel in a-priori high risk groups is unfortunately limited (Kuipers et al, 2007b), so definite conclusions on this issue cannot be drawn yet (Geerts et al, 2008). Nevertheless, the prevalence of air travel is high [two billion passengers flew in the year 2005 (Annual Review of Civil Aviation, 2005)]. Therefore, even a small increase in risk will have a major impact on the number of events. Consider that long distance air travel (more than 4 h) is associated with an absolute risk of venous thrombosis of 1 in 4600 passengers, and that 32% of all flights are long distance flights (Kuipers et al, 2007a). From these numbers it can be calculated that more than 140 000 air travellers per year experience venous thrombosis that is related to long distance air travel. Of note, this latter number merely serves as an example that when interpreting a risk, not only the strength of association (measured by a relative risk) or clinical impact (measured with an absolute risk), but also the prevalence of a certain exposure (in this case air travel) can have an impact on health status, in this case venous thrombotic disease, even when relative or absolute risks are low.