The early diagnosis of hypersensitivity lung disease (HLD) is important because of its progressive morbidity. It is often difficult to establish the diagnosis in the early stages because of the absence of defined symptoms. Case histories of 2 patients with pigeon breeder's HLD are reported. Both patients presented with unusual manifestations of the disease: one patient was asymptomatic but had an abnormal chest radiograph; the other patient had a normal chest radiograph but experienced occasional symptoms of exertional dyspnea. Both patients had abnormal pulmonary function and precipitin bands against pigeon serum. Following inhalation challenge with pigeon serum both patients developed fever and leukocytosis, but no significant pulmonary function response was observed. In both patients pulmonary function tests returned to near normal levels after corticosteroid therapy. Early detection of HLD may prevent progressive irreversible pulmonary damage. This requires a high index of clinical suspicion and appropriate screening tests. Inhalation challenge procedures are useful in establishing the diagnosis.