A 31-year-old man visits the pulmonologist after an episode of hemoptysis and dyspnea during an Alpine hiking trip. During the first few days of this trip, he had experienced difficulty keeping up with his traveling companions. When the group ascended to 2,700 m on the fifth day, he started to feel unwell and extremely fatigued. He noticed shortness of breath and a cough, first with white sputum and later with a fair amount of bloody sputum. That night, staying at the same altitude, he had difficulty sleeping and developed a headache. After descent to sea level he had no further complaints. He is a healthy nonsmoker who regularly exercises in a gym, although, from childhood on, his exercise capacity has been somewhat limited in comparison with his peers. He never used drugs or alcohol and takes no medication.