A 56-year-old man was admitted with cough, fever, myalgia, and arthralgia. Chest computed tomography demonstrated bilateral diffuse ground-glass opacities predominantly in the upper lungs. Subpleural non-segmental consolidation was observed in the late phase. Hypersensitivity pneumonitis was suspected, and an environmental provocation test with the incidental use of a home ultrasonic humidifier was positive. Unlike typical hypersensitivity pneumonitis, serum KL-6 levels were normal. Although several microorganisms were isolated from the humidifier water, there was no evidence for immune sensitization. We detected high amounts of endotoxin in the humidifier water, which may have contributed to the lung injury of this patient.