We describe a woman with hypersensitivity pneumonitis that was related to using a home ultrasonic humidifier. A micronodular infiltrate was seen in her chest radiograph. The inhalation challenge test was performed with the humidifier, and she exhibited a positive response. The cultures of the humidifier water grew Candida albicans, Rhodotorula spp., and Aspergillus spp. The test for precipitating antibodies against the humidifier water gave a positive response, and specific IgG, IgM, and IgA antibodies against extracts of A. fumigattis, C. albicans, and Rhodotorula spp. were demonstrated in the patient's serum by ELISA. A strong, dose-dependent inhibition of Rhodotorula IgG-ELISA by humidifier water was observed, suggesting that Rhodotorula might be the cause of hypersensitivity pneumonitis in this patient.