Three patients with mycosis fungoides who developed pulmonary abnormalities are described. These abnormalities are correlated with the findings at open lung biopsy and at autopsy. The roentgenographic changes consisted of bilateral, nodular pulmonary infiltrates in two patients, and interstitial infiltrate and plate-like atelectasis with hilar adenopathy in a third patient. Examination of lung biopsies and autopsy tissue revealed interstitial and/or intra-alveolar infiltrates composed of atypical lymphoid cells similar to those seem in the skin biopsies. The short survival after detection of an abnormal chest film suggests that dissemination of mycosis fungoides to the lung heralds a poor prognosis.