A 55-year-old man was admitted to our hospital because of severe dyspnea 30 minutes after inhalation of waterproofing spray. He had used the spray outdoors and had then smoked a cigarette with spray-contaminated fingers. Chest radiography and computed tomography (CT) revealed diffuse ground glass opacities in both lungs. In pulmonary function tests, the lungs showed a moderately decreased diffusing capacity and there was slight hypoxemia. Transbronchial lung biopsy specimens demonstrated extensive alveolitis and marked eosinophil migration. Without any specific treatment, the patient recovered clinically in 4 days. We speculated that acute lung injury in this patient may have been induced by not only direct inhalation of the waterproofing spray itself, but also by inhalation of spray by-products resulting from decomposition due to heat. When waterproofing spray is used, precautions should be taken to avoid both inhalation and heating of the fumes.