We studied the relationship of serum rheumatoid factor (RF) and certain other variables to lung diffusion capacity for carbon monoxide (DLCO) in patients with rheumatoid arthritis (RA). In non-smoking patients none of the examined variables correlated significantly to DLCO whereas in smokers RF showed a significant (p < 0.01) negative correlation in this respect. These results indicate that smoking in connection with RF may contribute to reduced DLCO in RA patients.