Hypersensitivity pneumonitis (HP) is an immunologically mediated inflammatory disorder of the pulmonary parenchyma which has been traditionally associated with the inhalation of organic dusts (farmer's lung, pigeon breeder's disease) and a variety of simple chemical compounds. More recently, the disease has been reported in association with the inhalation of aerosolized droplets in home and office settings. In Connecticut, the home and office, rather than the farm or factory, are the most common, although frequently unrecognized, environmental settings in which HP develops. We report here a case of a young, healthy woman who developed severe HP associated with the use of a portable home ultrasonic humidifier. This clinical entity should be suspected in the patient with a "recurrent viral illness" or "recurrent pneumonia." A review of the clinical and laboratory findings of humidifier-induced HP are discussed in relation to its pathogenesis, diagnosis, and treatment.