However, when the patients were stratified according to the severity of the disease to cases with moderate disease (stage II, n = 36) and with severe/very severe COPD (stage III/IV, n = 21), statistically significant results were obtained (Figure 2): in stage II, the non-smokers had significantly higher serum BDNF than the ex/current smokers (p = 0.026, Mann–Whitney U test) (Figure 2A). This evidence concerns the gene BDNF and chronic obstructive pulmonary disease.