Pulmonary hypertension (PH) can complicate myelofibrosis with myeloid metaplasia (MMM), may arise in the absence of evidence for thromboembolic disease and carries a grim prognosis. Four patients with MMM and severe symptomatic PH were treated with whole-lung external beam radiotherapy in a single fraction of 100 cGy. Within 72 h, each patient noted marked symptomatic improvement and had relief of hypoxia and reduction of oedema and/or ascites. Three of the four patients enjoyed an objective improvement in pulmonary artery systolic pressure as measured by transthoracic Doppler echocardiography. Low-dose lung radiotherapy may be a useful palliative tool for patients with MMM complicated by PH.