In the conventionally treated group of patients with chronic myelogenous leukemia (CML) the prognosis has been significantly improved by interferon-α (IFN-α). Several side effects in association with IFN-α treatment have been reported. Here we present the first case of a CML patient with reversible pulmonary artery hypertension (PAH) during IFN-α therapy. The patient received IFN-α-2b (up to 10 million U/day) for 6 months until he started to complain of dyspnea on exertion and an afebrile non-productive cough. An echocardiography and right heart catheterization showed signs of right heart failure with PAH (80 mmHg). A reduced carbon monoxide diffusion capacity and partial respiratory insufficiency were noted. Inflammatory markers were not elevated and pulmonary infiltrates could not be detected. Respiratory infections, thromboembolic causes or autoimmune diseases were carefully ruled out. IFN-α was suspected as causative agent, because experimental investigations in sheep showed that IFN-α can stimulate the thromboxane cascade which resulted in transient PAH. A reduced pulmonary diffusion capacity had been observed secondary to PAH. After discontinuation of IFN-α, our patient's clinical status improved rapidly. After 6 months the pulmonary artery pressure had returned to near normal values (35 mmHg) and the pulmonary diffusion capacity was normal. It took one year until the electrocardiogram reverted to the pre-IFN-α pattern. PAH should be included in the differential diagnosis of patients treated with IFN-α who complain of exertional dyspnea in the absence of inflammatory signs.