A 29-yr-old man with a fever of several days duration developed rapidly progressive bilateral lung disease with respiratory failure. An open lung biopsy showed extensive acute lung injury with atypical cells, but was not diagnostic. Subsequent cervical lymph node biopsy was consistent with malignant histiocytosis. There was a brief response to chemotherapy, but respiratory failure progressed. He died 34 days after hospital admission. Autopsy revealed extensive neoplastic involvement of both lungs as well as the spleen, cervical, mediastinal, and upper abdominal lymph nodes. This is the first report, to our knowledge, of malignant histiocytosis presenting primarily in the lung and progressing rapidly to respiratory failure. The possibility that this represents a neoplasm of the pulmonary macrophages is discussed.