Pulmonary infiltrates developed in a 40-year-old man while receiving arechine. The hyperplasia of mast cells, hyperlymphocytosis, a predominance of cytotoxic T-cells of the CD8 type with the very low CD4/CD8 ratio in BAL were observed. The chest roentgenogram findings resolved within a month after discontinuing arechine therapy. A hypersensitivity reaction with the onset of lung fibrosis due to arechine seemed to be the likely cause.