Of those 4,652 patients, there were 2,947 nonsmokers, 1,148 exsmokers, and 557 active smokers. There was no difference of APACHE II score between three groups of patients. The active smokers exhibited the highest incidence of ARDS (active smokers 5.4%, exsmokers 4.8%, and nonsmokers 3%, P = 0.003). There was no difference of 28-day mortality between the three groups of patients. Active smokers had the highest incidence of SIRS (active smokers 41%, exsmokers 37%, and nonsmokers 34%, P = 0.006). Compared with nonsmokers and exsmokers, active smokers had a longer SICU LOS (P < 0.01) and higher total SICU cost (P = 0.02). Patients who smoked more than 15 pack-years were 2.5 times more likely to develop ARDS than patients who smoked ≤15 pack-years (95% CI: 1.65 to 3.66, P < 0.001). In multivariate analysis we found that every 1 pack-year of cigarette smoking before admission to the SICU is associated with increased risk of new ARDS with a hazard ratio of 1.02 (95% CI: 1.01 to 1.02, P = 0.001) after adjustment for APACHE II score, age, gender, and chronic obstructive pulmonary disease.