The priming of bronchiolitis obliterans organising pneumonia by radiation therapy (RT) to the breast is now a well recognised syndrome. This study describes the occurrence of chronic eosinophilic pneumonia following RT after surgery for breast cancer in five female patients, with a mean age of 68 yrs (range 49-77). All patients had a history of asthma and/or allergy. At the onset of eosinophilic pneumonia, all patients were symptomatic. Chest radiograph showed pulmonary infiltrates, unilateral and limited to the irradiated lung in three patients, and bilateral in two. Pulmonary opacities were migratory in one patient. All patients had blood eosinophilia > 1.0 10(9.)L(-1) and/or eosinophilia > 40% at bronchoalveolar lavage differential cell count. The median time interval between the end of radiation therapy and the onset of eosinophilic pneumonia was 3.5 months (range 1-10). All patients rapidly improved with oral corticosteroids without sequelae. Relapse occurred in two patients after treatment withdrawal. Priming of alveolitis by radiation therapy to the breast might promote either bronchiolitis obliterans organising pneumonia or chronic eosinophilic pneumonia, with the latter depending on genetic or acquired characteristics of patients and/or further stimulation that may trigger a T-helper cell type 2 form of lymphocyte response, especially in patients with asthma or other atopic manifestations.