Although bronchial secretion is frequently observed in terminally ill cancer patients and can cause significant distress for both patients and family members, the pathophysiology is unclear. The primary aim of this study was to investigate the incidence and underlying etiologies of bronchial secretion. A multicenter, prospective, observational study was conducted on consecutive terminally ill patients with lung or abdominal malignancies. Primary physicians and nurses prospectively evaluated patients' symptoms. Of 310 patients enrolled, bronchial secretions were observed in 41% in the final 3 weeks, and oral/bronchial suctioning, with considerable distress, was required in 9%; bronchial secretions were severe in 4.5% of all patients. Multiple logistic regression analyses revealed that the determinants of the development of bronchial secretion were primary lung cancer, pneumonia, and dysphagia. There were no statistically significant effects of severity of peripheral edema and pleural effusion on development of bronchial secretions and requirement for oral/bronchial suctioning. Etiology-based classification of bronchial secretion is useful to identify the most suitable palliative treatments and to clarify treatment efficacy in each specific pathophysiology.