We examined the clinical characteristics of six patients with myelofibrosis secondary to myeloproliferative diseases whose clinical courses were complicated by pulmonary hypertension to determine possible causal links between the two disorders. Six patients (four male, two female), with diagnoses of myeloproliferative disease, myelofibrosis (one with polycythemia vera, three with agnogenic myeloid metaplasia, one with unclassified myeloproliferative syndrome, one with essential thrombocytosis), and pulmonary hypertension are presented. Measurement of the pulmonary artery pressure was performed by Doppler echocardiography in all patients and by right sided heart catheterization in four patients. The range of resting pulmonary artery systolic pressure was 35 to 47 mmHg above the mean right atrium by echocardiography. One patient had autopsy evidence of pulmonary myeloid metaplasia and interstitial fibrosis; another had acute leukemic infiltration of the lung parenchyma. All patients had thrombocytosis; symptomatology in one patient with marked thrombocytosis improved with plateletpheresis. Two patients suffered systemic thrombosis. All patients had severe hepatomegaly. Two patients had evidence of left ventricular dysfunction. The interval between the development of dyspnea and death was less than seven months in five of the patients. A causal link between pulmonary hypertension and myelofibrosis secondary to myeloproliferative diseases is suggested for each patient. Hematopoietic infiltration of the pulmonary parenchyma, portal hypertension, thrombocytosis, hypercoagulability, and left ventricular failure may account in part for the development of pulmonary hypertension in these patients. Patients with myelofibrosis and dyspnea should have Doppler echocardiography to evaluate pulmonary artery pressures.