Idiopathic bronchiolitis obliterans organizing pneumonia (BOOP) is characterized by air space inflammation and fibrosis of unknown origin. The pathogenesis of the inflammatory reaction and fibrosis in fibrotic lung disorders remains unclear; however, recent attention has focused on the potential role of the mast cell in the genesis of fibrosis. To determine whether mast cells are implicated in the pathogenesis of BOOP, mast cells were identified in BAL fluid and in transbronchial lung biopsy specimens from 11 patients affected by BOOP and 17 control subjects. Mast cells and tryptase were significantly increased in BAL fluid of patients with BOOP (p = 0.001 and p = 0.03, respectively). In lung tissue of patients with BOOP, there was an increased number of mast cells per square millimeter of lung tissue with respect to control group (p = 0.001). Seventy-three percent of mast cells were found in the alveolar septa, 18% within alveoli often plunged in organizing pneumonia, 4% among alveolar lining cells, and 6% along blood vessels. No mast cells were located within alveoli in control subjects. Mast cell degranulation was evident in lung tissue specimens of patients with BOOP but not in those of control subjects (p = 0.01). This study shows the importance of mast cells and mast cell activation in the pathogenesis of BOOP.