The purpose of this investigation was to quantify the effect of cigarette smoking on standard measures of lung function in patients with idiopathic pulmonary fibrosis (IPF). Our study population consisted of 73 patients in whom IPF had been clinically diagnosed; in 67% the diagnosis was confirmed by open lung biopsy. The average age was 63 yr; 62% were men, and 70% were either former or current cigarette smokers. Current cigarette smokers were found to have a greater percent predicted residual volume. Interestingly, in a univariate analysis, pack-years of cigarette smoking was found to be directly associated with increased measures of lung volumes (TLC, FRC, and RV) and diminished gas exchange (DLCO). Linear multivariate regression models demonstrated that current cigarette smokers have greater measures of RV and FRC and that increasing pack-years of cigarette smoking is associated with diminished gas exchange. Importantly, the FEV/FVC ratio was not significantly related to either smoking status or pack-years of cigarette smoking. Results from our study indicated that among patients with IPF, current cigarette smokers will tend to trap air (higher RV and FRC), and that cigarette smoking appears to adversely alter gas exchange. Moreover, IPF appears to reduce the likelihood of developing physiologic correlates of airflow obstruction among cigarette smokers. However, this does not imply that IPF prevents the development of cigarette-induced lung disease. In fact, the association between cigarette smoking and both increased lung volumes and diminished gas exchange suggests the presence of both emphysema and interstitial fibrosis.(ABSTRACT TRUNCATED AT 250 WORDS)