Diffuse panbronchiolitis (DPB), an important cause of progressive obstructive lung disease in the Far East, represents a distinctive sinobronchial syndrome with typical radiologic and histologic features. Human T-cell lymphotrophic virus (HTLV-1) is a retrovirus that clinically and experimentally suppresses T-cell function and immune responses. The clinical and immunologic features of DPB in HTLV-1 carriers are unclear, because DPB and HTLV-1 endemic areas around the world are mostly non-overlapping. However, both diseases are endemic in Japan. We present a patient with DPB positive for HTLV-1 whose cellular and humoral immune responses were markedly impaired. Six y after diagnosis of DPB, the patient developed respiratory failure and died in spite of treatment with clarithromycin.