A survey of 360 patients with various hematologic diseases revealed a high frequency of respiratory complications in patients with adult T-cell leukemia (ATL) compared to others. Among 29 patients with ATL, pulmonary complications were seen in 26 patients; leukemic pulmonary infiltration in 13, bleeding in 1, interstitial pneumonitis in 1, and pulmonary infection in 13. The incidence of Pneumocystis carinii and bacterial pneumonias were high despite adequate neutrophil count. Even in chronic and smoldering ATL, respiratory diseases were found in high frequency. Many of those were leukemic cell infiltration. In ten of 13 patients with pulmonary infiltration it occurred in the early stage and 6 of them were diagnosed as having "chronic lung disease" before the diagnosis of ATL. Its histology was accompanied by fibrosis in greater or lesser degree in almost all cases. Transbroncheal lung biopsy (TBLB) is of value in diagnosis (8 of 13 cases).