Smoking is a known risk factor for the development of several lung diseases, autoimmune diseases, and IgM rheumatoid factor (RF) in nonrheumatic persons. In patients with rheumatoid arthritis and IgM RF the diffusion capacity is decreased in smokers but not in nonsmokers. In the present study of patients with systemic sclerosis (SSc) the influence of smoking and IgM RF on the lung function was calculated. One hundred fifty-five persons with SSc had vital capacity (VC) and diffusing capacity (DLco) measured at least twice with at least 1-year interval as percents of predicted values according to gender, age, height, and weight. The yearly changes in VC and DLco were calculated, Delta VC and Delta DLco, respectively. IgM RF was measured at the beginning of the study. Smoking was defined as having ever smoked. Statistically significant deterioration of VC and DLco in patients with circulating IgM RF was found only in smokers or previous smokers, P = 0.007 and P = 0.01, respectively. These findings were confirmed by means of multiple regression analyses. The presence of IgM RF in smoking SSc patients is associated with deteriorating lung function. Whether this is a causal association and whether the presence of IgM RF in smoking patients with SSc actually confers an increased risk of pulmonary damage remains to be determined.