To investigate factors that determine bronchoalveolar lavage (BAL) cellularity in patients with idiopathic pulmonary fibrosis (IPF), we compared BAL cells in patients with IPF (n = 83) to both nonsmoking (n = 111) and smoking (n = 19) normal volunteers. Patients with IPF had higher concentrations of BAL total cells and alveolar macrophages than nonsmoking volunteers and more BAL neutrophils and eosinophils than normal volunteers regardless of smoking status. Among patients with IPF, the numbers of alveolar macrophages, neutrophils, or eosinophils were strongly associated with either smoking status or pack-years of cigarette smoking. In fact, after accounting for cigarette smoking, using multivariate analysis, the only additional factors that were found to be associated with BAL cellularity were age (macrophages and eosinophils) and the percent predicted forced expired volume in 1 s (neutrophils). Additional multivariate models failed to identify a significant relationship between BAL cellularity and either the type of immunosuppressive therapy or other physiological measures of lung function. We conclude that cigarette smoking strongly influences BAL cellularity in patients with IPF. These findings suggest that cigarette smoking may have a role in the pathogenesis of IPF or may adversely affect the prognosis in patients with IPF.