Three cases of allergic alveolitis due to indoor humdification systems are described. Thermoactinomyces vulgaris precipitins were detected in the serum of a 37-year-old female patient who had typical febrile attacks during exposure to cool-mist from a home humidifier. When the cause was detected and eliminated, the symptoms and signs disappeared and the woman's gas transfer factor improved from 56% to normal within six months. In a printing office a 60-year-old woman had had febrile attacks with cough for more than a year. The patient herself associated the respiratory disease with a cool-mist humidifier sometimes used at work. The water reservoir was heavily contaminated with amoebas (Amoeba proteus), which might have been the causative organisms in this case. Aspergillus fumigatus precipitins were found in the serum of a 53-year-old female printer with the clinical picture of occupational allergic alveolitis. The same organism was detected in the ambient air of the printing office.