A 50-year old female forest worker presented with the complaints of progressive exercise induced dyspnea and the history of a recent syncope. Right heart catheterisation revealed the diagnosis of severe precapillary pulmonary hypertension. Ventilation perfusion lung scan indicated the existence of multiple pulmonary emboli with high probability. However, pulmonary angiography did not confirm this finding. Computed tomography of the lung demonstrated a slight mosaic pattern of diffuse ground glass opacities. After video assisted thoracoscopy, the histological diagnosis of diffuse smooth muscle proliferation of the lung could be concluded from the removed lung tissue. Diffuse smooth muscle proliferation of the lung is a rarely diagnosed disease, however it should be considered in patients with suspected pulmonary embolism by ventilation perfusion scan but a normal finding in pulmonary angiography, who are suffering from otherwise not explained pulmonary hypertension.