A 24-year-old woman was admitted to our hospital because of pulmonary hypertension. Five years earlier, she had been given a diagnosis of systemic lupus erythematosus. The pulmonary hypertension was believed to have been caused by pulmonary vasculitis, because pulmonary angiography, nuclear perfusion scans, and axial magnetic resonance imaging of the pulmonary artery showed no evidence of pulmonary thromboembolism. Steroids, a calcium antagonist, and home oxygen therapy did not reduce the patient's pulmonary hypertension. The level of thromboxane B2, a stable metabolite of thromboxane A2, in the pulmonary artery was abnormally high (140 pg/ml). This suggested that vasoconstriction of the pulmonary artery and microthrombosis would cause continuous pulmonary hypertension. Beraprost sodium (120 micrograms/day, p.o.) was administered. This analogue of prostaglandin I2 is a potent relaxer of vascular smooth muscle, and it inhibits platelet aggregation. The pulmonary artery pressure was normal eight months after the start of therapy with beraprost sodium.