The prognosis for patients who have mixed connective tissue disease (MCTD) varies from a benign course to severe progressive disease. In approximately one third of patients the clinical symptoms go into long-term remission and the anti-U1 small nuclear ribonucleoprotein antibodies disappear. One third of patients have a severe, progressive disease course. Persistent morbidity often is attributable to arthritis, easy fatiguability, and dyspnea on exertion. The most severe clinical manifestation is pulmonary hypertension which contributes to premature death in patients who have MCTD. Pulmonary hypertension is associated with proliferative vascular abnormalities that involve small pulmonary vessels, rather than interstitial lung disease.