Radiotherapy improves locoregional control and survival of thoracic tumour patients. However, the associated exposure of normal tissues, often leads to side effects and possibly even reduces survival. Indeed, there is growing evidence that overall survival after radiotherapy for lung and oesophageal cancer is related to the radiation dose to heart and lungs. This suggests that thoracic radiotherapy causes mortality, which is currently not recognized as radiation-induced toxicity. So the question arises how to explain this treatment-related mortality. Interestingly, Ghobadi et al demonstrated in rats that thoracic irradiation can lead to pulmonary hypertension (PH). Histopathological analysis showed that radiation-induced PH closely resembles the pulmonary arterial hypertension (PAH) subtype. Moreover, in a clinical pilot study we confirmed early signs of PH including dose-dependent reductions in blood flow towards the lungs in radiotherapy patients. In general PH significantly affects survival. Moreover, the PAH subtype is the most-rapidly progressive and lethal subtype. However, medical treatment can significantly slow down PAH progression, providing opportunities for secondary prevention. Yet, hard evidence that radiation-induced PH is a clinically relevant phenomenon in patients treated for thoracic tumours, is lacking.