Serum levels of IL-18 in smokers (n = 28, 216.5±15.5 pg/mL) and COPD patients (n = 40, 235.4±13.2 pg/mL) were significantly (P<0.0001) higher than those in nonsmokers (n = 30, 113.7±13.5 pg/mL), as we previously reported [18]. Here, IL18 is linked to chronic obstructive pulmonary disease.