To further investigate the relationship between smoking and COPD with regard to SP-D levels, we subdivided our cohort into 5 groups consisting of healthy never smokers (n = 5), healthy former smokers (n = 3), former smokers with COPD (n = 8), healthy current smokers (n = 7), and current smokers with COPD (n = 12). Here, SFTPD is linked to chronic obstructive pulmonary disease.