Pulmonary involvement in mixed connective tissue disease (MCTD) is common, frequently severe, and is often clinically inapparent and variably responsive to corticosteroid/cyclophosphamide treatment. Serial pulmonary evaluation of patients with MCTD is important, since deterioration, as in the diffusing capacity over time, may alert the physician to the need for more invasive evaluation. Patients with a greater degree of overlap in rheumatological symptoms with an element of systemic sclerosis (PSS) may later develop severe disease. Nailfold capillary microscopy also may help in determining which patients will develop severe pulmonary involvement. Significant pulmonary hypertension occurs and cannot be accurately predicted on the basis of history, physical examination, pulmonary function tests, gallium scanning, or exercise testing. The characteristic pathological finding was intimal proliferation with medial muscular hypertrophy in the pulmonary arterioles. In contrast, pulmonary interstitial abnormalities were minimal, suggesting the proliferative vascular lesions are more closely associated with pulmonary hypertension in MCTD. Some patients develop rapidly progressive disease with varying response to corticosteroid and cytotoxic agents. More commonly, however, MCTD patients with long-term disabling disease, including pulmonary dysfunction, have had significant improvement with steroid and/or cyclophosphamide treatment, and clinical remission has occurred in 38% of the patients in this series.