A 37-year-old man who was engaged in golf club manufacturing for 10 years presented with dyspnea on exertion. His chest X-ray showed diffuse reticular shadow and the CT scan revealed mediastinal lymphadenopathy and diffuse reticular and ground-glass opacity with traction bronchiectasia. Laboratory findings were unremarkable except for elevated lysozyme and serum markers of interstitial pneumonia, such as KL-6. A lung function test showed restrictive ventilatory impairment and decreased diffusion capacity. The histological findings of surgical lung biopsy were fibrous thickening of alveolar walls with lymphocyte and plasma cell infiltration and scattered noncaseating granulomas. Realization that the patient had been exposed to beryllium alloy in golf club manufacturing for a couple of months aroused suspicion of chronic beryllium disease. A drug lymphocyte stimulation test using BeSO₄ as a suspected drug was performed in a reference laboratory and showed that BeSO₄ of concentration lower than or equal to 5 μg/ml stimulated patient's lymphocytes, but not his healthy control's, demonstrating that the patient was sensitized to beryllium, and the diagnosis was confirmed.