Ideally, the relationship between lung function and density should be studied in a large, unselected sample from the general population at a single center; however, this is not acceptable due to ethical issues related to exposing healthy individuals to the ionizing radiation of repeat CT scans, and in the real world single center studies are possible only in limited sample sizes. Realizing this, we decided to combine data from 2 large, observational studies with the aim of creating a unique sample covering the entire spectrum of COPD in heavy smokers, from individuals at risk of developing airflow limitation to patients with very severe COPD. In the 2 studies, CT was performed subsequent to inclusion, and participants were selected based on lung function only. Nevertheless, the combined sample not only covers a wide range of airflow limitation, but an equally wide range of CT lung densities and this is advantageous when studying the relationship between lung function and lung density. There is a risk of falsely inflating correlation when combining studies if any of the variables of interest reflects extremes; e.g., by combining super-healthy individuals and patients with very severe COPD. However, there is a significant overlap of both FEV1 and PD15 values in our cohorts as shown in Figure 1 and thus we do not think our findings are biased.