BACKGROUND: Analysis of bronchoalveolar lavage fluid has improved our understanding of the pathogenesis of asthma. Safety issues and access to expert resources limit this techniques as a research tool. Induced sputum is a non-invasive method of collecting airway fluid which is applicable to subjects with a range of severity of airflow obstruction. The method of sputum collection and processing differs between groups. A study was undertaken to compare induced sputum with bronchoscopically collected fluid. METHODS: Sixteen patients with mild stable asthma underwent both sputum induction and bronchoscopic examination with bronchial washings and bronchoalveolar lavage (BAL) in random order, with each procedure being separated by an interval of 12 days. Airway fluid was processed and stained for differential cell counting. RESULTS: Induced sputum was relatively rich in neutrophils and eosinophils compared with bronchial washings and BAL fluid (mean (SE) 1.3 (0.4)%, 5.0 (2.7)%, and 36.4 (3.7)% neutrophils and 0.6 (0.1)%, 1.6 (0.6)%, and 3.3 (1.1)% eosinophils in BAL fluid, bronchial washings, and induced sputum, respectively). The proportions of cells obtained at sputum induction correlated with those in bronchial washings but not BAL fluid (r = 0.6 and 0.7 for neutrophils and eosinophils, respectively, p < 0.05). By contrast, induced sputum had a lower proportion of lymphocytes and macrophages than bronchial washings or BAL fluid, without any correlation. CONCLUSION: Induced sputum is rich in neutrophils and eosinophils and poor in lymphocytes, suggesting an origin in the larger airways. Induced sputum adequately reflects the findings in fluid collected by direct bronchoscopy.