Tracheobronchial aspiration of gastric secretions has been suggested in published reports as a possible cause for idiopathic pulmonary fibrosis. Forty-eight of 131 patients with roentgenographic evidence of pulmonary fibrosis had no established etiologic diagnosis after individualized evaluations. They were prospectively studied by upper gastrointestinal series to determine the incidence of gastroesophageal reflux. The incidence of both hiatal hernia and reflux were statistically higher in the study group than in a group of 270 age-matched controls who had upper gastrointestinal series for the usual indications; (2) a subgroup of 15 patients who had pulmonary fibrosis and serologic evidence which suggested immune-mediated diseases; and (3) a subgroup of 23 patients with pulmonary fibrosis of established etiology. The patients in the study group could be further characterized by clinical and roentgenographic presentations, low maximum-mid-expiratory flow rates, and lung biopsies compatible with interstitial fibrosis. These observations and other cited evidence are supportive of the concept that repeated, small tracheobronchial aspirations of gastric acid secretions over a long period of time may cause interstitial pulmonary fibrosis.