Background: Growing evidence indicates that unilateral lung irradiation for breast cancer may “prime” the development of migratory lung infiltrates with histologic features of bronchiolitis obliterans organizing pneumonia. Objective: Our purpose was to evaluate the cytologic and immunocytologic features of bronchoalveolar lavage in this condition. Methods: We analyzed the profile bronchoalveolar lavage cell differentials and lymphocyte subpopulations of 11 women with bronchiolitis obliterans organizing pneumonia syndrome after radiation therapy for breast cancer in comparison to 9 healthy women. Results: The bronchoalveolar lavage analysis demonstrated a significant increase in the percentage of lymphocytes (36.7% ± 5.4% vs 8.6% ± 1.1%, P = .0002), neutrophils (3.8% ± 1.2% vs 0.6% ± 0.2%, P = .005), eosinophils (2.4% ± 1% vs 0.3% ± 0.1%, P = .01), and mast cells (1.4% ± 0.6% vs 0.1% ± 0.02%, P = .05) with a significant decrease in the percentage of macrophages (56.1% ± 6% vs 90.3% ± 1.4%, P = .0002) in patients with bronchiolitis obliterans organizing pneumonia compared with the control subjects. The percentage of CD3⁺ cells was significantly increased in patients with bronchiolitis obliterans organizing pneumonia (93.7% ± 1.3% vs 70.9% ± 4%, P = .0004), with a significant decrease in CD4⁺ cells (32.7% ± 4.7% vs 55.4% ± 2.6%, P = .002) and a significant increase in CD8⁺ cells (61.2% ± 4.8% vs 37.5% ± 2.9%, P = .003) in comparison to control subjects. The CD4/CD8 ratio was significantly reduced in patients with bronchiolitis obliterans organizing pneumonia compared with control subjects (0.6% ± 0.1% vs 1.5% ± 0.1%, P = .001). Conclusion: These data add to the view that unilateral lung irradiation for breast cancer may “prime” the development of a syndrome quite similar to idiopathic bronchiolitis obliterans organizing pneumonia. (J Allergy Clin Immunol 2000;105:239-44.)