Hot tub lung is a type of hypersensitivity pneumonitis caused by inhalational exposure to the Mycobacterium avium complex. We report the case of a 14-year-old boy presenting dyspnea with hypoxemia, whose medical history and clinical course helped make the diagnosis. Infectious causes were considered first, and antibiotics were initiated without success. Further questioning and explorations led to discussing a hypersensitivity pneumonitis diagnosis. Relapse after exposure to a hot tub enabled us to confirm hot tub lung. Hypersensitivity pneumonitis is a rare cause of pulmonary disorder, especially in children. It should be discussed with unusually severe and progressive cough or dyspnea. Further explorations should therefore be undertaken (CT, fibroscopy). Taking a precise medical history allows early diagnosis, leading to a quick withdrawal from the allergenic source and appropriate treatments.